ONE GAS, INC., 10-K filed on 2/23/2017
Annual Report
Document And Entity Information (USD $)
In Billions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Feb. 10, 2017
Jun. 30, 2016
Document Information [Line Items]
 
 
 
Entity Registrant Name
ONE Gas, Inc.  
 
 
Entity Central Index Key
0001587732 
 
 
Current Fiscal Year End Date
--12-31 
 
 
Entity Well-known Seasoned Issuer
Yes 
 
 
Entity Voluntary Filers
No 
 
 
Entity Current Reporting Status
Yes 
 
 
Entity Filer Category
Large Accelerated Filer 
 
 
Entity Public Float
 
 
$ 3.2 
Entity Common Stock, Shares Outstanding
 
52,283,788 
 
Document Fiscal Year Focus
2016 
 
 
Document Fiscal Period Focus
FY 
 
 
Document Type
10-K 
 
 
Amendment Flag
false 
 
 
Document Period End Date
Dec. 31, 2016 
 
 
STATEMENTS OF INCOME (USD $)
In Thousands, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Gross Margin
 
 
 
Revenues
$ 1,427,232 
$ 1,547,692 
$ 1,818,906 
Cost of natural gas
541,797 
705,959 
991,949 
Net margin
885,435 
841,733 
826,957 
Operating expenses
 
 
 
Operations and maintenance
417,142 
414,476 
420,686 
Depreciation and amortization
143,829 
133,023 
125,722 
General taxes
55,344 
55,105 
55,255 
Total operating expenses
616,315 
602,604 
601,663 
Operating income
269,120 
239,129 
225,294 
Other income
1,447 
263 
1,625 
Other expense
(1,490)
(2,813)
(2,949)
Interest expense, net
(43,739)
(44,570)
(45,842)
Income before income taxes
225,338 
192,009 
178,128 
Income taxes
(85,243)
(72,979)
(68,338)
Net income
140,095 
119,030 
109,790 
Net Income (Loss) Available to Common Stockholders, Diluted
$ 140,095 
$ 119,030 
$ 109,790 
Earnings per share
 
 
 
Basic
$ 2.67 
$ 2.26 
$ 2.10 
Diluted
$ 2.65 
$ 2.24 
$ 2.07 
Average shares (thousands)
 
 
 
Basic
52,453 
52,578 
52,364 
Diluted
52,963 
53,254 
52,946 
Dividends declared per share of stock
$ 1.40 
$ 1.20 
$ 0.84 
STATEMENTS OF COMPREHENSIVE INCOME STATEMENTS OF COMPREHENSIVE INCOME (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Net income
$ 140,095 
$ 119,030 
$ 109,790 
Other Comprehensive Income (Loss), Net of Tax [Abstract]
 
 
 
Change in pension and other postretirement benefit plans liability, net of tax of $197, $(483), and $1,244, respectively
(314)
773 
(1,781)
Other comprehensive loss
(314)
773 
(1,781)
Comprehensive income
$ 139,781 
$ 119,803 
$ 108,009 
STATEMENTS OF COMPREHENSIVE INCOME (Parentheticals) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
STATEMENTS OF COMPREHENSIVE INCOME [Abstract]
 
 
 
Pension and other postretirement benefit plans, Tax
$ 197 
$ (483)
$ 1,244 
BALANCE SHEETS (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2016
Dec. 31, 2015
Property, plant and equipment
 
 
Property, plant and equipment
$ 5,404,168 
$ 5,132,682 
Accumulated depreciation and amortization
1,672,548 
1,620,771 
Net property, plant and equipment
3,731,620 
3,511,911 
Current assets
 
 
Cash and cash equivalents
14,663 
2,433 
Accounts receivable, net
290,944 
216,343 
Materials and supplies
34,084 
33,325 
Income tax receivable
1,397 
38,877 
Natural gas in storage
125,432 
142,153 
Regulatory assets
83,146 
32,925 
Other current assets
19,257 
16,789 
Total current assets
568,923 
482,845 
Goodwill and other assets
 
 
Regulatory assets
440,522 
435,863 
Goodwill
157,953 
157,953 
Other assets
43,773 
46,193 
Total goodwill and other assets
642,248 
640,009 
Total assets
4,942,791 
4,634,765 
Equity and long-term debt
 
 
Common stock, $0.01 par value: authorized 250,000,000 shares; issued 52,598,005 shares and outstanding 52,283,260 shares at December 31, 2016; issued 52,598,005 shares and outstanding 52,259,224 shares at December 31, 2015
526 
526 
Paid-in capital
1,749,574 
1,764,875 
Retained earnings
161,021 
95,046 
Accumulated other comprehensive income (loss)
(4,715)
(4,401)
Treasury stock, at cost: 338,781 shares at December 31, 2015 and 314,745 shares at December 31, 2016
(18,126)
(14,491)
Total equity
1,888,280 
1,841,555 
Long-term debt, excluding current maturities, and net of issuance costs of $8,851 and $9,645, respectively
1,192,446 
1,191,660 
Total equity and long-term debt
3,080,726 
3,033,215 
Current liabilities
 
 
Current maturities of long-term debt
7 
7 
Notes payable
145,000 
12,500 
Accounts payable
131,988 
107,482 
Accrued interest
18,854 
18,873 
Accrued taxes other than income
42,571 
37,249 
Accrued liabilities
22,931 
31,470 
Customer deposits
61,209 
60,325 
Regulatory liabilities
11,922 
24,615 
Other current liabilities
9,451 
11,700 
Total current liabilities
443,933 
304,221 
Deferred credits and other liabilities [Abstract]
 
 
Deferred income taxes
1,038,568 
951,785 
Employee benefit obligations
303,507 
272,309 
Other deferred credits
76,057 
73,235 
Total deferred credits and other liabilities
1,418,132 
1,297,329 
Commitments and contingencies
   
   
Total liabilities and equity
$ 4,942,791 
$ 4,634,765 
BALANCE SHEETS BALANCE SHEETS Parenthetical (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Statement of Financial Position [Abstract]
 
 
Common Stock, Par or Stated Value Per Share
$ 0.01 
$ 0.01 
Common Stock, Shares Authorized
250,000,000 
250,000,000 
Common Stock, Shares, Issued
52,598,005 
52,598,005 
Common Stock, Shares, Outstanding
52,283,260 
52,259,224 
Treasury Stock, Shares
314,745 
338,781 
Debt Issuance Cost
$ 8,851 
$ 9,645 
STATEMENTS OF CASH FLOWS (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Net income
$ 140,095 
$ 119,030 
$ 109,790 
Adjustments to reconcile net income to net cash provided by operating activities:
 
 
 
Depreciation and amortization
143,829 
133,023 
125,722 
Deferred income taxes
86,788 
63,789 
49,935 
Share-based compensation expense
11,219 
9,187 
7,613 
Provision for doubtful accounts
5,427 
4,520 
7,195 
Changes in assets and liabilities:
 
 
 
Accounts receivable
(80,028)
105,886 
23,044 
Materials and supplies
(759)
(5,814)
10,868 
Income Tax Receivable
37,480 
4,923 
(43,800)
Natural gas in storage
16,721 
43,147 
(19,172)
Asset removal costs
(53,430)
(51,608)
(47,125)
Accounts payable
27,596 
(59,635)
(6,881)
Accrued interest
(19)
1 
18,743 
Accrued taxes other than income
5,322 
(7,493)
12,316 
Accrued Liabilities
(8,539)
5,451 
21,228 
Customer deposits
884 
322 
2,643 
Regulatory assets and liabilities
(49,472)
50,658 
30,067 
Employee benefit obligation
(25,666)
(15,033)
(10,102)
Other assets and liabilities
24,119 
(6,147)
(45,421)
Cash provided by operating activities
281,567 
394,207 
246,663 
Investing activities
 
 
 
Capital expenditures
(309,071)
(294,320)
(297,103)
Other
492 
0 
0 
Cash used in investing activities
(308,579)
(294,320)
(297,103)
Financing activities
 
 
 
Borrowings (repayment) on notes payable, net
132,500 
(29,500)
42,000 
Repurchase of common stock
(24,066)
(24,122)
0 
Issuance of debt, net of discounts
0 
0 
1,199,994 
Long-term debt financing costs
0 
0 
(11,087)
Cash payment to ONEOK upon separation
0 
0 
(1,130,000)
Issuance of common stock
4,017 
7,051 
2,001 
Dividends paid
(73,209)
(62,826)
(43,696)
Cash provided by financing activities
39,242 
(109,397)
59,212 
Change in cash and cash equivalents
12,230 
(9,510)
8,772 
Cash and cash equivalents at beginning of period
2,433 
11,943 
3,171 
Cash and cash equivalents at end of period
14,663 
2,433 
11,943 
Supplemental cash flow information:
 
 
 
Cash paid for interest, net of amounts capitalized
42,129 
42,980 
21,066 
Cash (received) paid for income taxes, net
$ (35,702)
$ (5,423)
$ 44,603 
STATEMENT OF CHANGES IN EQUITY (USD $)
In Thousands, except Share data
Total
Retained Earnings [Member]
Owner's Net Investment [Member]
Accumulated Other Comprehensive Income (Loss) [Member]
Treasury Stock [Member]
Common Stock [Member]
Additional Paid-in Capital [Member]
Equity, beginning balance at Dec. 31, 2013
$ 1,239,023 
$ 0 
$ 1,239,023 
$ 0 
$ 0 
$ 0 
$ 0 
Shares Issued, beginning balance at Dec. 31, 2013
 
 
 
 
 
100 
 
Common Stock, Dividends, Declared, Annualized Basis
$ 0.84 
 
 
 
 
 
 
Net income
109,790 
84,214 
25,576 
0 
0 
0 
0 
Other comprehensive loss
(1,781)
0 
0 
(1,781)
0 
0 
0 
Net transfers from ONEOK
481,086 
0 
484,479 
(3,393)
0 
0 
0 
Reclassification of Owner's investment to paid-in capital
0 
0 
(1,749,078)
0 
0 
0 
1,749,078 
Net issuance of common stock at the separation, Shares
 
 
 
 
 
51,941,136 
 
Issuance of common stock at the separation, value
0 
0 
0 
0 
0 
520 
(520)
Common stock issued, shares
 
 
 
 
 
142,623 
 
Common stock issued, value
9,615 
0 
0 
0 
0 
1 
9,614 
Common stock dividends
(43,696)
(44,320)
0 
0 
0 
0 
624 
Equity, ending balance at Dec. 31, 2014
1,794,037 
39,894 
0 
(5,174)
0 
521 
1,758,796 
Shares Issued, ending balance at Dec. 31, 2014
 
 
 
 
 
52,083,859 
 
Common Stock, Dividends, Declared, Annualized Basis
$ 1.20 
 
 
 
 
 
 
Net income
119,030 
119,030 
0 
0 
0 
0 
0 
Other comprehensive loss
773 
0 
0 
773 
0 
0 
0 
Repurchase of common stock
(24,122)
0 
0 
0 
(24,122)
0 
0 
Common stock issued, shares
 
 
 
 
 
514,146 
 
Common stock issued, value
14,663 
0 
0 
0 
9,631 
5 
5,027 
Common stock dividends
(62,826)
(63,878)
0 
0 
0 
0 
1,052 
Equity, ending balance at Dec. 31, 2015
1,841,555 
95,046 
0 
(4,401)
(14,491)
526 
1,764,875 
Shares Issued, ending balance at Dec. 31, 2015
 
 
 
 
 
52,598,005 
 
Common Stock, Dividends, Declared, Annualized Basis
$ 1.40 
 
 
 
 
 
 
Net income
140,095 
140,095 
0 
0 
0 
0 
0 
Other comprehensive loss
(314)
0 
0 
(314)
0 
0 
0 
Repurchase of common stock
(24,066)
0 
0 
0 
(24,066)
0 
0 
Common stock issued, shares
 
 
 
 
 
0 
 
Common stock issued, value
4,219 
0 
0 
0 
20,431 
0 
(16,212)
Common stock dividends
(73,209)
(74,120)
0 
0 
0 
0 
911 
Equity, ending balance at Dec. 31, 2016
$ 1,888,280 
$ 161,021 
$ 0 
$ (4,715)
$ (18,126)
$ 526 
$ 1,749,574 
Shares Issued, ending balance at Dec. 31, 2016
 
 
 
 
 
52,598,005 
 
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Notes)
SIGNIFICANT ACCOUNTING POLICIES
1.
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Organization and Nature of Operations - Prior to January 31, 2014, ONE Gas was a wholly owned subsidiary of ONEOK and comprised its former natural gas distribution business. On January 31, 2014, ONEOK distributed one share of our common stock for every four shares of ONEOK common stock held by ONEOK shareholders of record as of the close of business on January 21, 2014, the record date of the distribution. At the close of business on January 31, 2014, we became an independent, publicly traded company as a result of the distribution. Our common stock began trading “regular-way” under the ticker symbol “OGS” on the NYSE on February 3, 2014.

We provide natural gas distribution services to more than 2 million customers through our divisions in Oklahoma, Kansas and Texas through Oklahoma Natural Gas, Kansas Gas Service and Texas Gas Service, respectively. We serve residential, commercial, industrial and transportation customers in all three states. In addition, we also provide natural gas distribution services to wholesale and public authority customers.

Basis of Presentation - Prior to our separation from ONEOK, our financial statements were derived from ONEOK’s financial statements, which included its natural gas distribution business as if we, for accounting purposes, had been a separate company for all periods presented. The assets and liabilities in the financial statements have been reflected on a historical basis. The financial statements for the period prior to the separation also includes expense allocations for certain corporate functions historically performed by ONEOK, including allocations of general corporate expenses related to executive oversight, accounting, treasury, tax, legal, information technology and other services. We believe our assumptions underlying the financial statements, including the assumptions regarding the allocation of general corporate expenses from ONEOK, are reasonable. However, the financial statements may not include all of the actual expenses that would have been incurred by us and may not reflect our results of operations, financial position and cash flows had we been a separate publicly traded company during the period presented prior to the separation.

All financial information presented after the separation represents the results of operations, financial position and cash flows of ONE Gas. Accordingly:
•
Our Statements of Income and Comprehensive Income for the year ended December 31, 2014, consist of the results of ONE Gas for the eleven months ended December 31, 2014, and the results of ONE Gas Predecessor for the one month ended January 31, 2014.
•
Our Statement of Cash Flows for the year ended December 31, 2014, consists of the results of ONE Gas for the eleven months ended December 31, 2014, and the results of ONE Gas Predecessor for the one month ended January 31, 2014.
•
Our Statement of Equity for the year ended December 31, 2014, consists of both the activity for ONE Gas Predecessor prior to January 31, 2014, and the activity for ONE Gas completed in connection with, and subsequent to, the separation on January 31, 2014.

The financial statements include the accounts of the natural gas distribution business as set forth in “Organization and Nature of Operations” above. All significant balances and transactions between our divisions have been eliminated.

Use of Estimates - The preparation of our financial statements and related disclosures in accordance with GAAP requires us to make estimates and assumptions with respect to values or conditions that cannot be known with certainty that affect the reported amount of assets and liabilities, and the disclosure of contingent assets and liabilities at the date of the financial statements. These estimates and assumptions also affect the reported amounts of revenue and expenses during the reporting period. Items that may be estimated include, but are not limited to, the economic useful life of assets, fair value of assets and liabilities, provisions for doubtful accounts receivable, unbilled revenues for natural gas delivered but for which meters have not been read, natural gas purchased but for which no invoice has been received, provision for income taxes, including any deferred tax valuation allowances, the results of litigation and various other recorded or disclosed amounts.

We evaluate these estimates on an ongoing basis using historical experience and other methods we consider reasonable based on the particular circumstances. Nevertheless, actual results may differ significantly from the estimates. Any effects on our financial position or results of operations from revisions to these estimates are recorded in the period when the facts that give rise to the revision become known.

Fair Value Measurements - We define fair value as the price that would be received from the sale of an asset or the transfer of a liability in an orderly transaction between market participants at the measurement date. We use the market and income approaches to determine the fair value of our assets and liabilities and consider the markets in which the transactions are executed. We measure the fair value of a group of financial assets and liabilities consistent with how a market participant would price the net risk exposure at the measurement date.

Fair Value Hierarchy - At each balance sheet date, we utilize a fair value hierarchy to classify fair value amounts recognized or disclosed in our financial statements based on the observability of inputs used to estimate such fair value. The levels of the hierarchy are described below:
•
Level 1 - Unadjusted quoted prices in active markets for identical assets or liabilities;
•
Level 2 - Significant observable pricing inputs other than quoted prices included within Level 1 that are, either directly or indirectly, observable as of the reporting date. Essentially, this represents inputs that are derived principally from or corroborated by observable market data; and
•
Level 3 - May include one or more unobservable inputs that are significant in establishing a fair value estimate. These unobservable inputs are developed based on the best information available and may include our own internal data.

We recognize transfers into and out of the levels as of the end of each reporting period.

Determining the appropriate classification of our fair value measurements within the fair value hierarchy requires management’s judgment regarding the degree to which market data is observable or corroborated by observable market data. We categorize derivatives for which fair value is determined using multiple inputs within a single level, based on the lowest level input that is significant to the fair value measurement in its entirety. See Note 7 for additional information regarding our fair value measurements.

Cash and Cash Equivalents - Cash equivalents consist of highly liquid investments, which are readily convertible into cash and have original maturities of three months or less.

Revenue Recognition - For regulated deliveries of natural gas, we read meters and bill customers on a monthly cycle. We recognize revenues upon the delivery of the natural gas commodity or services rendered to customers. The billing cycles for customers do not necessarily coincide with the accounting periods used for financial reporting purposes. Revenues are accrued for natural gas delivered and services rendered to customers, but not yet billed. Accrued unbilled revenue is based on a percentage estimate of amounts unbilled each month, which is dependent upon a number of factors, some of which require management’s judgment. These factors include customer consumption patterns and the impact of weather on usage. The amounts of accrued unbilled natural gas sales revenues at December 31, 2016 and 2015, were $143.2 million and $109.6 million, respectively.

We collect and remit other taxes on behalf of governmental authorities, and we record these amounts in accrued taxes other than income in our Balance Sheets on a net basis.

Cost of Natural Gas - Net margin is comprised of total revenues less cost of natural gas.  Cost of natural gas includes commodity purchases, fuel, storage, transportation and other gas purchase costs recovered through our cost of natural gas regulatory mechanisms and does not include an allocation of general operating costs or depreciation and amortization.  In addition, our cost of natural gas regulatory mechanisms provide a method of recovering natural gas costs on an ongoing basis without a profit. Therefore, although our revenues will fluctuate with the cost of gas that we purchase, net margin is not affected by fluctuations in the cost of natural gas. See Note 8 regulatory assets and liabilities for additional discussion of purchased gas cost recoveries.

Accounts Receivable - Accounts receivable represent valid claims against nonaffiliated customers for natural gas sold or services rendered, net of allowances for doubtful accounts. We assess the creditworthiness of our customers. Those customers who do not meet minimum standards are required to provide security, including deposits and other forms of collateral, when appropriate. With more than 2 million customers across three states, we are not exposed materially to a concentration of credit risk. We maintain an allowance for doubtful accounts based upon factors surrounding the credit risk of customers, historical trends, consideration of the current credit environment and other information. In Oklahoma, Kansas and most jurisdictions we serve in Texas, we are able to recover natural gas costs related to doubtful accounts through purchased-gas cost adjustment mechanisms. At December 31, 2016 and 2015, our allowance for doubtful accounts was $4.2 million and $3.5 million, respectively.

Inventories - Natural gas in storage is maintained on the basis of weighted-average cost. Natural gas inventories that are injected into storage are recorded in inventory based on actual purchase costs, including storage and transportation costs. Natural gas inventories that are withdrawn from storage are accounted for in our purchased-gas cost adjustment mechanisms at the weighted-average inventory cost.

Materials and supplies inventories are stated at the lower of weighted-average cost or net realizable value.

Derivatives and Risk Management Activities - We record all derivative instruments at fair value, with the exception of normal purchases and normal sales that are expected to result in physical delivery. The accounting for changes in the fair value of a derivative instrument depends on whether it has been designated and qualifies as part of a hedging relationship and, if so, the reason for holding it, or if regulatory rulings require a different accounting treatment.

If certain conditions are met, we may elect to designate a derivative instrument as a hedge of exposure to changes in fair values or cash flows.

The table below summarizes the various ways in which we account for our derivative instruments and the impact on our financial statements:
 
 
Recognition and Measurement
Accounting Treatment
 
Balance Sheet
 
Income Statement
Normal purchases and
normal sales
-
Fair value not recorded
-
Change in fair value not recognized in earnings
Mark-to-market
-
Recorded at fair value
-
Change in fair value recognized in, and
recoverable through, the purchased-gas cost adjustment mechanisms

We have not elected to formally designate any of our derivative instruments as hedges. Gains or losses associated with the fair value of commodity derivative instruments entered into by us are included in, and recoverable through, the purchased-gas cost adjustment mechanisms.

See Note 7 for additional information regarding our fair value measurements and hedging activities using derivatives.

Property, Plant and Equipment - Our properties are stated at cost, which includes direct construction costs such as direct labor, materials, burden and AFUDC. Generally, the cost of our property retired or sold, plus removal costs, less salvage, is charged to accumulated depreciation. Gains and losses from sales or retirement of an entire operating unit or system of our properties are recognized in income. Maintenance and repairs are charged directly to expense.

AFUDC represents the cost of borrowed funds used to finance construction activities. We capitalize interest costs during the construction or upgrade of qualifying assets. Capitalized interest is recorded as a reduction to interest expense.

Our properties are depreciated using the straight-line method over their estimated useful lives. Generally, we apply composite depreciation rates to functional groups of property having similar economic circumstances. We periodically conduct depreciation studies to assess the economic lives of our assets. These depreciation studies are completed as a part of our regulatory proceedings, and the changes in economic lives, if applicable, are implemented prospectively when the new rates are effective. Changes in the estimated economic lives of our property, plant and equipment could have a material effect on our financial position, results of operations or cash flows.

Property, plant and equipment on our Balance Sheets includes construction work in process for capital projects that have not yet been placed in service and therefore are not being depreciated. Assets are transferred out of construction work in process when they are substantially complete and ready for their intended use.

See Note 9 for additional information regarding our property, plant and equipment.

Impairment of Goodwill and Long-Lived Assets - We assess our goodwill for impairment at least annually as of July 1. Our goodwill impairment analysis performed in 2016, 2015 and 2014, utilized a qualitative assessment and did not result in any impairment indicators. Subsequent to July 1, 2016, no event has occurred indicating that it is more likely than not that our fair value is less than our carrying value of our net assets.

As part of our goodwill impairment test, we first assess qualitative factors (including macroeconomic conditions, industry and market considerations, cost factors and overall financial performance) to determine whether it is more likely than not that our fair value is less than our carrying amount. If further testing is necessary, we perform a two-step impairment test for goodwill. In the first step, an initial assessment is made by comparing our fair value with our book value, including goodwill. If the fair value is less than the book value, an impairment is indicated, and we must perform a second test to measure the amount of the impairment. In the second test, we calculate the implied fair value of the goodwill by deducting the fair value of all tangible and intangible net assets from the fair value determined in step one of the assessment. If the carrying value of the goodwill exceeds the implied fair value of the goodwill, we will record an impairment charge.

To estimate our fair value, we use two generally accepted valuation approaches, an income approach and a market approach, using assumptions consistent with a market participant’s perspective. Under the income approach, we use anticipated cash flows over a period of years plus a terminal value and discount these amounts to their present value using appropriate discount rates. Under the market approach, we apply acquisition multiples to forecasted cash flows. The acquisition multiples used are consistent with historical market transactions. The forecasted cash flows are based on average forecasted cash flows over a period of years.

We assess our long-lived assets for impairment whenever events or changes in circumstances indicate that an asset’s carrying amount may not be recoverable. An impairment is indicated if the carrying amount of a long-lived asset exceeds the sum of the undiscounted future cash flows expected to result from the use and eventual disposition of the asset. If an impairment is indicated, we record an impairment loss equal to the difference between the carrying value and the fair value of the long-lived asset. We determined that there were no asset impairments in 2016, 2015 or 2014.

Regulation - We are subject to the rate regulation and accounting requirements of the OCC, KCC, RRC and various municipalities in Texas. We follow the accounting and reporting guidance for regulated operations. During the ratemaking process, regulatory authorities set the framework for what we can charge customers for our services and establish the manner that our costs are accounted for, including allowing us to defer recognition of certain costs and permitting recovery of the amounts through rates over time, as opposed to expensing such costs as incurred. Examples include weather normalization, unrecovered purchased-gas costs, pension and postemployment benefit costs and ad-valorem taxes. This allows us to stabilize rates over time rather than passing such costs on to the customer for immediate recovery. Actions by regulatory authorities could have an effect on the amount recovered from rate payers. Any difference in the amount recoverable and the amount deferred is recorded as income or expense at the time of the regulatory action. A write-off of regulatory assets and costs not recovered may be required if all or a portion of the regulated operations have rates that are no longer:
•
established by independent regulators;
•
designed to recover the specific entity’s costs of providing regulated services; and
•
set at levels that will recover our costs when considering the demand and competition for our services.

See Note 8 for additional information regarding our regulatory assets and liabilities disclosures.

Pension and Other Postemployment Employee Benefits - We have defined benefit retirement plans covering eligible employees. We also sponsor welfare plans that provide other postemployment medical and life insurance benefits to eligible employees who retire with at least five years of service. To calculate the costs and liabilities related to our plans, we utilize an outside actuarial consultant, which uses statistical and other factors to anticipate future events. These factors include assumptions about the discount rate, expected return on plan assets, rate of future compensation increases, age and mortality and employment periods. In determining the projected benefit obligations and costs, assumptions can change from period to period and may result in material changes in the cost and liabilities we recognize.

Income Taxes - Deferred income taxes are recorded for the difference between the financial statement and income tax basis of assets and liabilities and carryforward items, based on income tax laws and rates existing at the time the temporary differences are expected to reverse. The effect on deferred taxes of a change in tax rates is deferred and amortized for operations regulated by the OCC, KCC, RRC and various municipalities in Texas, if, as a result of an action by a regulator, it is probable that the effect of the change in tax rates will be recovered from or returned to customers through future rates. We continue to amortize previously deferred investment tax credits for ratemaking purposes over the periods prescribed by our regulators.

A valuation allowance for deferred tax assets is recognized when it is more likely than not that some or all of the benefit from the deferred tax asset will not be realized. To assess that likelihood, we use estimates and judgment regarding our future taxable income, as well as the jurisdiction in which such taxable income is generated, to determine whether a valuation allowance is required. Such evidence can include our current financial position, our results of operations, both actual and forecasted, the reversal of deferred tax liabilities, as well as the current and forecasted business economics of our industry. We had no valuation allowance at December 31, 2016 and 2015.

We utilize a more-likely-than-not recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position that is taken or expected to be taken in a tax return. We reflect penalties and interest as part of income tax expense as they become applicable for tax provisions that do not meet the more-likely-than-not recognition threshold and measurement attribute. There were no material uncertain tax positions at December 31, 2016 and 2015. See Note 12 for additional information regarding income taxes.

Asset Retirement Obligations - Asset retirement obligations represent legal obligations associated with the retirement of long-lived assets that result from the acquisition, construction, development and/or normal use of the asset. Certain long-lived assets that comprise our natural gas distribution systems, primarily our pipeline assets, are subject to agreements or regulations that give rise to an asset retirement obligation for removal or other disposition costs associated with retiring the assets in place upon the discontinued use of the natural gas distribution system. We recognize the fair value of a liability for an asset retirement obligation in the period when it is incurred if a reasonable estimate of the fair value can be made. We are not able to estimate reasonably the fair value of the asset retirement obligations for portions of our assets because the settlement dates are indeterminable given our expected continued use of the assets with proper maintenance. We expect our natural gas distribution systems will continue in operation as long as natural gas supply and demand for natural gas distribution service exists. Based on the widespread use of natural gas for heating and cooking activities by residential and commercial customers in our service areas, management expects supply and demand to exist for the foreseeable future.

In accordance with long-standing regulatory treatment, we collect through rates the estimated costs of removal on certain regulated properties through depreciation expense, with a corresponding credit to accumulated depreciation and amortization. These removal costs collected through our rates include costs attributable to legal and nonlegal removal obligations; however, the amounts collected that are in excess of these nonlegal asset-removal costs incurred are accounted for as a regulatory liability for financial reporting purposes. Historically, with the exception of the regulatory authority in Kansas, the regulatory authorities that have jurisdiction over our regulated operations have not required us to quantify or disclose this amount; rather, these costs are addressed prospectively in depreciation rates and are set in each general rate order. We have made an estimate of our regulatory liability using current rates since the last general rate order in each of our jurisdictions if the removal costs collected have exceeded our removal cost incurred; however, for financial reporting purposes, significant uncertainty exists regarding the future disposition of this regulatory liability, pending, among other issues, clarification of regulatory intent. We continue to monitor the regulatory requirements, and the liability may be adjusted as more information is obtained. We record the estimated asset removal obligation in noncurrent liabilities in other deferred credits on our Balance Sheets. To the extent this estimated liability is adjusted, such amounts will be reclassified between accumulated depreciation and amortization and other deferred credits and therefore will not have an impact on earnings.

Contingencies - Our accounting for contingencies covers a variety of business activities, including contingencies for legal and environmental exposures. We accrue these contingencies when our assessments indicate that it is probable that a liability has been incurred or an asset will not be recovered and an amount can be estimated reasonably. We expense legal fees as incurred and base our legal liability estimates on currently available facts and our estimates of the ultimate outcome or resolution. Accruals for estimated losses from environmental remediation obligations generally are recognized no later than the completion of a remediation feasibility study. Recoveries of environmental remediation costs from other parties are recorded as assets when their receipt is deemed probable. Actual results may differ from our estimates resulting in an impact, positive or negative, on earnings. See Note 13 for additional information regarding contingencies.

Share-Based Payments - We expense the fair value of share-based payments net of estimated forfeitures. We estimate forfeiture rates based on historical forfeitures under our share-based payment plans.

Earnings per share - Basic EPS is based on net income and is calculated based upon the daily weighted-average number of common shares outstanding during the periods presented. Also, this calculation includes fully vested stock awards that have not yet been issued as common stock. Diluted EPS includes the above, plus unvested stock awards granted under our compensation plans, but only to the extent these instruments dilute earnings per share.

Segments - We operate in one reportable business segment: regulated public utilities that deliver natural gas to residential, commercial, industrial, wholesale, public authority and transportation customers. We define reportable business segments as components of an organization for which discrete financial information is available and operating results are evaluated on a regular basis by the chief operating decision maker (CODM) in order to assess performance and allocate resources. Our CODM is our Chief Executive Officer (CEO). Characteristics of our organization that were relied upon in making this determination include the similar nature of services we provide, the functional alignment of our organizational structure, and the reports that are regularly reviewed by the CODM for the purpose of assessing performance and allocating resources. Our management is functionally aligned and centralized, with performance evaluated based upon results of the entire distribution business. Capital allocation decisions are driven by asset integrity management, operating efficiency, growth opportunities and government relocations, not geographic location or regulatory jurisdiction.

In 2016, 2015 and 2014, we had no single external customer from which we received 10 percent or more of our gross revenues.

Treasury Stock - We record treasury stock purchases at cost, which includes incremental direct transaction costs. Amounts are recorded as reductions in equity in our Balance Sheets. We record the reissuance of treasury stock at our weighted average cost of treasury shares recorded in equity in our Balance Sheets.

Recently Issued Accounting Standards Update - In January 2017, the FASB issued ASU 2017-04, “Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment,” which simplifies how an entity is required to test goodwill for impairment by eliminating Step 2 of the goodwill test, where the measurement of a goodwill impairment loss was determined by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. Upon adoption, a goodwill impairment will be the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill.  This new guidance is required for our interim and annual reports for periods beginning after December 15, 2019, and early adoption is permitted. We do not expect this guidance to have a material impact on our financial statements and will adjust our goodwill testing procedures accordingly upon adoption.

In March 2016, the FASB issued ASU 2016-09, “Improvements to Employee Share-Based Payment Accounting,” which includes various new aspects to simplify how share-based payments are accounted for and presented in the financial statements. The new standard modifies several aspects of the accounting and reporting for employee share-based payments and related tax accounting impacts, including the presentation in the statements of operations and cash flows. We will adopt this new guidance in the first quarter of 2017. Prospectively, we will record excess tax expenses or benefits in income tax expense. We will record a cumulative-effect increase of $11.0 million to retained earnings, with an offset to a deferred tax asset, as of the beginning of the reporting period in 2017 for excess tax benefits earned prior to January 1, 2017. We will continue our use of the estimation method to account for share unit awards forfeitures rather than actual forfeitures. We will adopt the classification of cash flows for changes in excess tax benefits prospectively in operating activities, and employer withholding shares for tax-withholding purposes for employees retrospectively in investing activities in our statement of cash flows.

In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842),” which prescribes recognizing lease assets and liabilities on the balance sheet and includes disclosure of key information about leasing arrangements.  A modified retrospective transition approach is required for leases existing at the time of adoption. We are evaluating our population of leases, analyzing lease agreements, and holding meetings with cross-divisional teams to determine the potential impact of this accounting standard on our financial position or results of operations and the transition approach we will utilize. This new guidance is required for our interim and annual reports for periods beginning after December 15, 2018, and early adoption is permitted.

In October 2015, the FASB issued ASU 2015-17, “Balance Sheet Classification of Deferred Taxes,” to simplify reporting of deferred taxes. The new guidance requires all deferred tax assets and liabilities, along with any related valuation allowance, be classified as noncurrent on the balance sheet. This guidance is required to be adopted for our interim and annual reports for periods beginning after December 15, 2016, but early adoption is permitted. We have adopted this guidance early to simplify our financial reporting process, have applied it prospectively for the period beginning October 1, 2015, and it did not have a material impact on our financial statements. Prior periods were not retrospectively adjusted.

In August 2015, the FASB issued ASU 2015-15, “Interest-Imputation of Interest (Subtopic 835-30),” which specifically addresses the presentation and subsequent measurement of debt issuance costs associated with line of credit arrangements. We adopted this guidance in the first quarter 2016, and it did not have an impact on our financial position or results of operations.

In April 2015, the FASB issued ASU 2015-03, “Interest-Imputation of Interest,” which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability. We adopted this guidance in the first quarter of 2016, and have applied the changes retrospectively to all periods presented. We have presented such amounts as a direct deduction from the face amount of our long-term debt, rather than in other assets as a deferred charge in our Balance Sheets. Amortization of the debt issuance costs continues to be reported as interest expense in our Statements of Income.

In April 2015, the FASB issued ASU 2015-05, “Intangibles-Goodwill and Other-Internal-Use Software,” which helps entities evaluate the accounting for fees paid by a customer in a cloud computing arrangement. We adopted this guidance prospectively in the first quarter of 2016, and it did not have a material impact on our financial position or results of operations.

In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers,” which clarifies and converges the revenue recognition principles under GAAP and International Financial Reporting Standards. In July 2015, FASB delayed the effective date for one year. We have substantially completed evaluating all of our sources of revenue to determine the potential effect on our financial position, results of operations and cash flows. We continue to monitor accounting task forces and the FASB for additional implementation guidance related to: (1) the accounting for funds received from third parties to partially or fully reimburse the cost of construction of an asset; (2) the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers; and (3) the accounting for alternative revenue programs, such as performance-based ratemaking, that may impact the final conclusions of our evaluation. Until these items are resolved, we cannot determine the effect the new guidance will have on our financial position, results of operations, cash flows, business processes or the transition method we will utilize to adopt the new guidance. We are required to adopt this new guidance for our interim and annual reports beginning with the first quarter 2018.
CREDIT FACILITY AND SHORT-TERM NOTES PAYABLE (Notes)
Short-term Debt [Text Block]
2.
CREDIT FACILITY AND SHORT-TERM NOTES PAYABLE

The ONE Gas Credit Agreement contains certain financial, operational and legal covenants. Among other things, these covenants include maintaining ONE Gas’ total debt-to-capital ratio of no more than 70 percent at the end of any calendar quarter. The ONE Gas Credit Agreement also contains customary affirmative and negative covenants, including covenants relating to liens, indebtedness of subsidiaries, investments, changes in the nature of business, fundamental changes, transactions with affiliates, burdensome agreements, and use of proceeds. In the event of a breach of certain covenants by ONE Gas, amounts outstanding under the ONE Gas Credit Agreement may become due and payable immediately. At December 31, 2016, our total debt-to-capital ratio was 41 percent and we were in compliance with all covenants under the ONE Gas Credit Agreement.

The ONE Gas Credit Agreement includes a $50 million sublimit for the issuance of standby letters of credit and also features an option to request an increase in the size of the facility to an aggregate of $1.2 billion from $700 million by either commitments from new lenders or increased commitments from existing lenders. Borrowings made under the facility are available for general corporate purposes. The ONE Gas Credit Agreement contains provisions for an applicable margin rate and an annual facility fee, both of which adjust with changes in our credit rating. Based on our current credit ratings, borrowings, if any, will accrue interest at LIBOR plus 79.5 basis points, and the annual facility fee is 8 basis points.

We have a commercial paper program under which we may issue unsecured commercial paper up to a maximum amount of $700 million to fund short-term borrowing needs. The maturities of the commercial paper notes may vary but may not exceed 270 days from the date of issue. The commercial paper notes are sold generally at par less a discount representing an interest factor.

The ONE Gas Credit Agreement is available to repay the commercial paper notes, if necessary. Amounts outstanding under the commercial paper program reduce the borrowing capacity under the ONE Gas Credit Agreement.

At December 31, 2016, we had $145.0 million of commercial paper and $1.5 million in letters of credit issued under the ONE Gas Credit Agreement, with no borrowings and $553.5 million of remaining credit available under the ONE Gas Credit Agreement. The weighted-average interest rate on our commercial paper was 0.95 percent and 0.70 percent at December 31, 2016 and 2015, respectively.
LONG-TERM DEBT (Notes)
Long-term Debt [Text Block]
3.
LONG-TERM DEBT

In January 2014, we issued senior notes, consisting of $300 million of 2.07 percent senior notes due 2019, $300 million of 3.61 percent senior notes due 2024 and $600 million of 4.658 percent senior notes due 2044. The indenture governing our Senior Notes includes an event of default upon the acceleration of other indebtedness of $100 million or more. Such events of default would entitle the trustee or the holders of 25 percent in the aggregate principal amount of the outstanding Senior Notes to declare those senior notes immediately due and payable in full.

We may redeem our Senior Notes at par, plus accrued and unpaid interest to the redemption date, starting one month, three months, and six months, respectively, before their maturity dates. Prior to these dates, we may redeem these Senior Notes, in whole or in part, at a redemption price equal to the principal amount, plus accrued and unpaid interest and a make-whole premium. The redemption price will never be less than 100 percent of the principal amount of the respective note plus accrued and unpaid interest to the redemption date. Our Senior Notes are senior unsecured obligations, ranking equally in right of payment with all of our existing and future unsecured senior indebtedness.
EQUITY (Notes)
Stockholders' Equity Note Disclosure [Text Block]
4.
EQUITY

Preferred Stock - At December 31, 2016, we had 50 million, $0.01 par value, authorized shares of preferred stock available. We have not issued or established any classes or series of shares of preferred stock.

Common Stock - At December 31, 2016, we had approximately 197.7 million shares of authorized common stock available for issuance.

Treasury Shares - We purchase treasury shares to be used to offset shares issued under our employee and non-employee director equity compensation and employee stock purchase plans. Our Board of Directors established an annual limit of $20 million of treasury stock purchases, exclusive of funds received through the dividend reinvestment and employee stock purchase plans. Stock purchases may be made in the open market or in private transactions at times, and in amounts that we deem appropriate. There is no guarantee as to the exact number of shares that we purchase, and we can terminate or limit the program at any time.

Dividends Declared - In January 2017, we declared a dividend of $0.42 per share ($1.68 per share on an annualized basis) for shareholders of record on February 24, 2017, payable March 10, 2017.
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Notes)
Comprehensive Income (Loss) Note [Text Block]
5.
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)

The following table sets forth the balance in accumulated other comprehensive income (loss) for the period indicated:
 
 
Accumulated Other Comprehensive Income (Loss)
 
 
(Thousands of dollars)
January 1, 2015
 
$
(5,174
)
Pension and other postemployment benefit plans obligations
 
 
Other comprehensive income (loss) before reclassification, net of tax of $(130)
 
209

Amounts reclassified from accumulated other comprehensive income (loss), net of tax of $(353)
 
564

Other comprehensive income (loss)
 
773

December 31, 2015
 
(4,401
)
Pension and other postemployment benefit plans obligations
 
 
Other comprehensive income (loss) before reclassification, net of tax of $486
 
(776
)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax of $(289)
 
462

Other comprehensive income (loss)
 
(314
)
December 31, 2016
 
$
(4,715
)


The following table sets forth the effect of reclassifications from accumulated other comprehensive income (loss) on our Statements of Income for the period indicated:
Details about Accumulated Other Comprehensive Income
 
Year Ended December 31,
Affected Line Item in the
(Loss) Components
 
2016
 
2015
 
2014
Statements of Income
 
 
(Thousands of dollars)
 
Pension and other postemployment benefit plan obligations (a)
 
 
 
 
 
 
 
Amortization of net loss
 
$
40,912

 
$
47,494

 
$
34,169

 
Amortization of unrecognized prior service cost
 
(3,316
)
 
(1,962
)
 
(1,211
)
 
 
 
37,596

 
45,532

 
32,958

 
Regulatory adjustments (b)
 
(36,845
)
 
(44,615
)
 
(32,445
)
 
 
 
751

 
917

 
513

Income before income taxes
 
 
(289
)
 
(353
)
 
(198
)
Income tax expense
Total reclassifications for the period
 
$
462

 
$
564

 
$
315

Net income
(a) These components of accumulated other comprehensive income (loss) are included in the computation of net periodic benefit cost. See Note 11 for additional information regarding our net periodic benefit cost.
(b) Regulatory adjustments represent pension and other postemployment benefit costs expected to be recovered through rates and are deferred as part of our regulatory assets. See Note 8 for additional information regarding our regulatory assets and liabilities.
EARNINGS PER SHARE (Notes)
Earnings Per Share [Text Block]
6.
EARNINGS PER SHARE

The following tables set forth the computation of basic and diluted EPS from continuing operations for the periods indicated:
 
Year Ended December 31, 2016
 
Income
 
Shares
 
Per Share
Amount
 
(Thousands, except per share amounts)
Basic EPS Calculation
 
 
 
 
 
Net income available for common stock
$
140,095

 
52,453

 
$
2.67

Diluted EPS Calculation
 

 
 

 
 

Effect of dilutive securities
—

 
510

 
 

Net income available for common stock and common stock equivalents
$
140,095

 
52,963

 
$
2.65


 
Year Ended December 31, 2015
 
Income
 
Shares
 
Per Share
Amount
 
(Thousands, except per share amounts)
Basic EPS Calculation
 
 
 
 
 
Net income available for common stock
$
119,030

 
52,578

 
$
2.26

Diluted EPS Calculation
 
 
 

 
 

Effect of dilutive securities
—

 
676

 
 

Net income available for common stock and common stock equivalents
$
119,030

 
53,254

 
$
2.24


 
Year Ended December 31, 2014
 
Income
 
Shares
 
Per Share
Amount
 
(Thousands, except per share amounts)
Basic EPS Calculation
 
 
 
 
 
Net income available for common stock
$
109,790

 
52,364

 
$
2.10

Diluted EPS Calculation
 

 
 

 
 

Effect of dilutive securities
—

 
582

 
 

Net income available for common stock and common stock equivalents
$
109,790

 
52,946

 
$
2.07

DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Notes)
Fair Value Disclosures
7.
DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS

Derivative Instruments - At December 31, 2016, we held purchased natural gas call options for the heating season ending March 2017, with total notional amounts of 14.3 Bcf, for which we paid premiums of $5.4 million, and which had a fair value of $6.5 million. At December 31, 2015, we held purchased natural gas call options for the heating season ended March 2016, with total notional amounts of 17.0 Bcf, for which we paid premiums of $5.8 million, and which had a fair value of $0.4 million. The premiums paid and any cash settlements received are recorded as part of our unrecovered purchased-gas costs in current regulatory assets as these contracts are included in, and recoverable through, the purchased-gas cost adjustment mechanisms. Additionally, changes in fair value associated with these contracts are deferred as part of our unrecovered purchased-gas costs in our Balance Sheets. Our natural gas call options are classified as Level 1 as fair value amounts are based on unadjusted quoted prices in active markets including NYMEX-settled prices. There were no transfers between levels for the periods presented.

Other Financial Instruments - The approximate fair value of cash and cash equivalents, accounts receivable and accounts payable is equal to book value, due to the short-term nature of these items. Our cash and cash equivalents are comprised of bank and money market accounts, and are classified as Level 1.

Short-term notes payable and commercial paper are due upon demand and, therefore, the carrying amounts approximate fair value and are classified as Level 1. The book value of our long-term debt, including current maturities, was $1.2 billion at both December 31, 2016 and 2015. The estimated fair value of our long-term debt, including current maturities, was $1.2 billion at both December 31, 2016 and 2015. The estimated fair value of our Senior Notes was determined using quoted market prices, and are considered Level 2.
REGULATORY ASSETS AND LIABILITIES (Notes)
Schedule of Regulatory Assets and Liabilities
8.
REGULATORY ASSETS AND LIABILITIES

The table below presents a summary of regulatory assets, net of amortization, and liabilities for the periods indicated:
 
 
 
 
December 31, 2016
 
 
Remaining Recovery Period
 
Current
 
Noncurrent
 
Total
 
 
 
 
(Thousands of dollars)
Under-recovered purchased-gas costs
 
1 year
 
$
29,901

 
$
—

 
$
29,901

Pension and other postemployment benefit costs
 
See Note 11
 
31,498

 
427,448

 
458,946

Weather normalization
 
1 year
 
17,661

 
—

 
17,661

Reacquired debt costs
 
11 years
 
812

 
8,108

 
8,920

Other
 
1 to 22 years
 
3,274

 
4,966

 
8,240

Total regulatory assets, net of amortization
 
 
 
83,146

 
440,522

 
523,668

Over-recovered purchased-gas costs
 
1 year
 
(10,154
)
 
—

 
(10,154
)
Ad-valorem tax
 
1 year
 
(1,768
)
 
—

 
(1,768
)
Total regulatory liabilities
 
 
 
(11,922
)
 
—

 
(11,922
)
Net regulatory assets and liabilities
 
 
 
$
71,224

 
$
440,522

 
$
511,746


 
 
 
 
December 31, 2015
 
 
Remaining Recovery Period
 
Current
 
Noncurrent
 
Total
 
 
 
 
(Thousands of dollars)
Under-recovered purchased-gas costs
 
1 year
 
$
13,336

 
$
—

 
$
13,336

Pension and other postemployment benefit costs
 
See Note 11
 
15,670

 
425,175

 
440,845

Weather normalization
 
1 year
 
2,198

 
—

 
2,198

Reacquired debt costs
 
12 years
 
812

 
8,919

 
9,731

Other
 
1 to 23 years
 
909

 
1,769

 
2,678

Total regulatory assets, net of amortization
 
 
 
32,925

 
435,863

 
468,788

Accumulated removal costs (a)
 
up to 50 years
 
—

 
(9,032
)
 
(9,032
)
Over-recovered purchased-gas costs
 
1 year
 
(22,884
)
 
—

 
(22,884
)
Ad-valorem tax
 
1 year
 
(1,731
)
 
—

 
(1,731
)
Total regulatory liabilities
 
 
 
(24,615
)
 
(9,032
)
 
(33,647
)
Net regulatory assets and liabilities
 
 
 
$
8,310

 
$
426,831

 
$
435,141

(a) Included in other deferred credits in our Balance Sheets.

Regulatory assets on our Balance Sheets, as authorized by the various regulatory authorities, are probable of recovery. Base rates are designed to provide a recovery of cost during the period rates are in effect but do not generally provide for a return on investment for amounts we have deferred as regulatory assets. All of our regulatory assets recoverable through base rates are subject to review by the respective regulatory authorities during future rate proceedings. We are not aware of any evidence that these costs will not be recoverable through either rate riders or base rates, and we believe that we will be able to recover such costs, consistent with our historical recoveries.

Purchased-gas costs represent the natural gas costs that have been over- or under-recovered from customers through the purchased-gas cost adjustment mechanisms, and includes natural gas utilized in our operations and premiums paid and any cash settlements received from our purchased natural gas call options.

We amortize reacquired debt costs in accordance with the accounting guidelines prescribed by the OCC and KCC.

Weather normalization represents revenue over- or under-recovered through the WNA rider in Kansas. This amount is deferred as a regulatory asset or liability for a 12-month period. Kansas Gas Service then applies an adjustment to the customers’ bills for 12 months to refund the over-collected revenue or bill the under-collected revenue.

Ad-valorem tax represents an increase or decrease in Kansas Gas Service’s taxes above or below the amount approved in a rate case. This amount is deferred as a regulatory asset or liability for a 12-month period. Kansas Gas Service then applies an adjustment to the customers’ bills for 12 months to refund the over-collected revenue or bill the under-collected revenue.

Recovery through rates resulted in amortization of regulatory assets of approximately $3.8 million, $1.6 million and $6.4 million for the years ended December 31, 2016, 2015 and 2014, respectively.

We collect, through our rates, the estimated costs of removal on certain regulated properties through depreciation expense, with a corresponding credit to accumulated depreciation and amortization. These removal costs are nonlegal obligations; however, the amounts collected that are in excess of these nonlegal asset-removal costs incurred are accounted for as a regulatory liability. We have made an estimate of our regulatory liability using current rates since the last general rate order in each of our jurisdictions if the removal costs collected have exceeded our removal costs incurred. We record the estimated nonlegal asset-removal obligation in noncurrent liabilities in other deferred credits on our Balance Sheets.

In January 2016, as a result of our rate case in Oklahoma, we recorded a regulatory asset of $2.4 million to recover certain information technology costs incurred as a result of our separation from ONEOK in 2014, which will be recovered over four years.
PROPERTY, PLANT AND EQUIPMENT (Notes)
PROPERTY, PLANT AND EQUIPMENT
9.
PROPERTY, PLANT AND EQUIPMENT

The following table sets forth our property, plant and equipment by property type, for the periods indicated:
 
 
December 31,
 
December 31,
 
 
2016
 
2015
 
 
(Thousands of dollars)
Natural gas distribution pipelines and related equipment
 
$
4,321,429

 
$
4,114,090

Natural gas transmission pipelines and related equipment
 
481,953

 
462,654

General plant and other
 
530,459

 
498,906

Construction work in process
 
70,327

 
57,032

Property, plant and equipment
 
5,404,168

 
5,132,682

Accumulated depreciation and amortization
 
(1,672,548
)
 
(1,620,771
)
Net property, plant and equipment
 
$
3,731,620

 
$
3,511,911



We compute depreciation expense by applying composite, straight-line rates of 2.0 percent to 3.0 percent that were approved by various regulatory authorities.

We recorded capitalized interest of $3.6 million, $2.6 million and $2.5 million for the years ended December 31, 2016, 2015 and 2014, respectively. We incurred liabilities for construction work in process and asset removal costs that had not been paid at December 31, 2016, 2015 and 2014 of $11.9 million, $15.0 million and $7.0 million, respectively. Such amounts are not included in capital expenditures on the Statements of Cash Flows.
SHARE-BASED PAYMENTS (Notes)
Disclosure of Compensation Related Costs, Share-based Payments
10.
SHARE-BASED PAYMENTS

The ONE Gas Equity Compensation Plan (ECP) provides for the granting of stock-based compensation, including incentive stock options, nonstatutory stock options, stock bonus awards, restricted stock awards, restricted stock unit awards, performance stock awards and performance unit awards to eligible employees and the granting of stock awards to nonemployee directors. We have reserved 2.8 million shares of common stock for issuance under the ECP. At December 31, 2016, we had approximately 1.1 million shares available for issuance under the ECP, which reflect shares issued and estimated shares expected to be issued upon vesting of outstanding awards granted under the plan, less forfeitures. The plan allows for the deferral of awards granted in stock or cash, in accordance with Internal Revenue Code section 409A requirements.

Compensation cost expensed for our share-based payment plans was $7.0 million, net of tax benefits of $4.3 million, for 2016, $5.7 million, net of tax benefits of $3.5 million, for 2015, and $7.0 million, net of tax benefits of $4.4 million, for 2014.

Restricted Stock Unit Awards - We have granted restricted stock unit awards to key employees that vest over a service period of generally three years and entitle the grantee to receive shares of our common stock. Restricted stock unit awards granted accrue dividend equivalents in the form of additional restricted stock units prior to vesting. Restricted stock unit awards are measured at fair value as if they were vested and issued on the grant date, reduced by expected dividend payments for awards that do not accrue dividends and adjusted for estimated forfeitures. Compensation expense is recognized on a straight-line basis over the vesting period of the award. A forfeiture rate of 3 percent per year based on historical forfeitures under our share-based payment plans is used.

Performance Stock Unit Awards - We have granted performance stock unit awards to key employees. The shares of common stock underlying the performance stock units vest at the expiration of a service period of generally three years if certain performance criteria are met by us as determined by the Executive Compensation Committee of the Board of Directors. Upon vesting, a holder of performance stock units is entitled to receive a number of shares of common stock equal to a percentage (0 percent to 200 percent) of the performance stock units granted, based on our total shareholder return over the vesting period, compared with the total shareholder return of a peer group of other utilities over the same period.

If paid, the outstanding performance stock unit awards entitle the grantee to receive shares of our common stock. The outstanding performance stock unit awards are equity awards with a market-based condition, which results in the compensation expense for these awards being recognized on a straight-line basis over the requisite service period, provided that the requisite service period is fulfilled, regardless of when, if ever, the market condition is satisfied. The performance stock unit awards granted accrue dividend equivalents in the form of additional performance stock units prior to vesting. The fair value of these performance stock units was estimated on the grant date based on a Monte Carlo model. The compensation expense on these awards will only be adjusted for changes in forfeitures. A forfeiture rate of 3 percent per year based on historical forfeitures under our share-based payment plans was used.

Restricted Stock Unit Award Activity

As of December 31, 2016, there was $2.8 million of total unrecognized compensation costs related to the nonvested restricted stock unit awards, which is expected to be recognized over a weighted-average period of 1.7 years. The following tables set forth activity and various statistics for restricted stock unit awards outstanding under the respective plans for the period indicated:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested December 31, 2015
 
231,258

 
$
32.59

Granted
 
42,935

 
$
58.30

Vested
 
(77,033
)
 
$
23.76

Forfeited
 
(2,260
)
 
$
38.04

Nonvested December 31, 2016
 
194,900

 
$
41.68

 
 
2016
 
2015
 
2014
Weighted-average grant date fair value (per share)
 
$
58.30

 
$
41.40

 
$
33.19

Fair value of shares granted (thousands of dollars)
 
$
2,503

 
$
3,141

 
$
3,149



The fair value of restricted stock vested was $4.5 million and $6.5 million in 2016 and 2015, respectively.

Performance Stock Unit Award Activity

As of December 31, 2016, there was $4.8 million of total unrecognized compensation cost related to the nonvested performance stock unit awards, which is expected to be recognized over a weighted-average period of 1.7 years. The following tables set forth activity and various statistics related to our performance stock unit awards and the assumptions used by us in the valuations of the 2016, 2015 and 2014 grants at the grant date:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested December 31, 2015
 
439,250

 
$
27.35

Granted
 
74,395

 
$
64.06

Vested
 
(221,882
)
 
$
15.11

Forfeited
 
(2,952
)
 
$
41.44

Nonvested December 31, 2016
 
288,811

 
$
46.06

 
 
2016
 
2015
 
2014
 
Volatility (a)
 
18.20%
 
15.90%
 
18.40%
 
Dividend yield
 
2.40%
 
2.90%
 
3.37%
 
Risk-free interest rate
 
0.91%
 
1.10%
 
0.67%
 
(a) - Volatility based on historical volatility over three years using daily stock price observations of our peer utilities.
 
 
 
2016
 
2015
 
2014
Weighted-average grant date fair value (per share)
 
$
64.06

 
$
44.48

 
$
35.98

Fair value of shares granted (thousands of dollars)
 
$
4,766

 
$
4,486

 
$
4,462



The fair value of performance stock vested was $19.5 million and $23.5 million in 2016 and 2015, respectively.

Employee Stock Purchase Plan

We have reserved a total of 700 thousand shares of common stock for issuance under our Employee Stock Purchase Plan (the ESPP).  Subject to certain exclusions, all employees who work at least 20 hours per week are eligible to participate in the ESPP.  Employees can choose to have up to 10 percent of their annual base pay withheld to purchase our common stock, subject to terms and limitations of the plan. The purchase price of the stock is 85 percent of the lower of the average market price of our common stock on the grant date or exercise date. Approximately 41 percent, 40 percent and 36 percent of employees participated in the plan in 2016, 2015 and 2014, respectively, and purchased 83,431 shares at $54.51 in 2016, 51,092 shares at $36.15 in 2015, and 51,418 shares at $32.29 in 2014. Compensation expense, before taxes, was $1.4 million, $1.3 million and $0.4 million in 2016, 2015 and 2014, respectively.

Employee Stock Award Program

Under the Employee Stock Award Program, we issue, for no monetary consideration, one share of our common stock to all eligible employees when the per-share closing price of our common stock on the NYSE closes for the first time at or above each $1.00 increment above $34. The total number of shares of our common stock authorized for issuance under this program is 125,000. Shares issued to employees under this program during 2016, 2015 and 2014 totaled 50,573, 23,506 and 35,324, respectively, leaving 15,603 shares for future awards. Compensation expense, before taxes, related to the Employee Stock Award Program was $3.0 million, $1.1 million and $2.5 million for 2016, 2015 and 2014, respectively.
EMPLOYEE BENEFIT PLANS (Notes)
Pension and Other Postemployment Benefits Disclosure [Text Block]
11.
EMPLOYEE BENEFIT PLANS

Retirement and Other Postemployment Benefit Plans

Retirement Plans - We have a defined benefit pension plan covering nonbargaining-unit employees hired before January 1, 2005, and certain bargaining-unit employees hired before December 15, 2011. Nonbargaining unit employees hired after December 31, 2004; employees represented by Local No. 304 of the International Brotherhood of Electrical Workers (IBEW) hired on or after July 1, 2010; employees represented by the United Steelworkers hired on or after December 15, 2011; and employees who accepted a one-time opportunity to opt out of the defined benefit pension plan are covered by a profit-sharing plan. Certain employees of the Texas Gas Service division are entitled to benefits under a frozen cash-balance pension plan. In addition, we have a supplemental executive retirement plan for the benefit of certain officers. No new participants in the supplemental executive retirement plan have been approved since 2005, and it was formally closed to new participants as of January 1, 2014. We fund our defined benefit pension costs at a level needed to maintain or exceed the minimum funding levels required by the Employee Retirement Income Security Act of 1974, as amended, and the Pension Protection Act of 2006. Pension expense was $32.0 million, $38.0 million and $27.1 million in 2016, 2015 and 2014, respectively.

Other Postemployment Benefit Plans - We sponsor health and welfare plans that provide postemployment medical and life insurance benefits to certain employees who retire with at least five years of service. The postemployment medical plan is contributory based on hire date, age and years of service, with retiree contributions adjusted periodically, and contains other cost-sharing features such as deductibles and coinsurance. Other postemployment benefit expense was $2.6 million, $5.0 million and $5.9 million in 2016, 2015 and 2014, respectively, prior to regulatory deferrals.

Plan Amendments - In October 2015, we announced to certain pre-65 participants in our postemployment medical plans a change from a self-insured postemployment medical plan to a plan providing participants an annual benefit that would allow them to select coverage on a healthcare exchange beginning January 1, 2017. As a result, we remeasured the respective plan assets and liabilities, which resulted in a reduction in benefit obligations of our postemployment benefit plan of $11.9 million in the fourth quarter of 2015.

In September 2016, due to uncertain market conditions with health insurance exchange providers, we elected not to move the eligible pre-65 participants in our postemployment medical plans to a healthcare exchange. As a result, we remeasured the respective plan assets and benefit obligations, effective September 30, 2016. In the fourth quarter of 2016, we further amended our other postemployment medical plan to allow certain participants access to reimbursable retirement accounts. The net impact of these plan amendments in 2016 was a $483 thousand increase in our other postemployment benefit plan obligation.
 
Actuarial Assumptions - The following table sets forth the weighted-average assumptions used to determine benefit obligations for pension and postemployment benefits for the periods indicated:
 
 
December 31,
 
 
2016
 
2015
Discount rate - pension plans
 
4.30%
 
4.75%
Discount rate - other postemployment plans
 
4.20%
 
4.75%
Compensation increase rate
 
3.25% - 3.40%
 
3.35% - 3.40%

The following table sets forth the weighted-average assumptions used by us to determine the periodic benefit costs for the periods indicated:
 
 
Nine Months Ended September 30,
 
Three Months Ended December 31,
 
Years Ended December 31,
 
 
2016
 
2016
 
2015
 
 
2014
Discount rate - pension plans
 
4.75%
 
4.75%
 
4.25%/4.75%
(a)
 
5.25%
Discount rate - other postemployment plans
 
4.75%
 
3.75%
 
4.25%/4.75%
(a)
 
5.00%
Expected long-term return on plan assets - pension plans
 
7.75%
 
7.75%
 
7.75%
 
 
7.75%
Expected long-term return on plan assets - other postemployment plans
 
8.00%
 
7.75%
 
7.75%
 
 
7.75%
Compensation increase rate
 
3.35% - 3.40%
 
3.35% - 3.40%
 
3.30% - 3.50%
 
 
3.35% - 3.50%

(a) Discount rate for the nine months ended September 30, 2015, and three months ended December 31, 2015, respectively.

We determine our overall expected long-term rate of return on plan assets, based on our review of historical returns and economic growth models. At December 31, 2016, we updated our assumed mortality rates to incorporate the new set of mortality tables issued by the Society of Actuaries in October 2016.

We determine our discount rates annually.  We estimate our discount rate based upon a comparison of the expected cash flows associated with our future payments under our defined benefit pension and other postemployment obligations to a hypothetical bond portfolio created using high-quality bonds that closely match expected cash flows.  Bond portfolios are developed by selecting a bond for each of the next 60 years based on the maturity dates of the bonds.  Bonds selected to be included in the portfolios are only those rated by Moody’s as AA- or better and exclude callable bonds, bonds with less than a minimum issue size, yield outliers and other filtering criteria to remove unsuitable bonds.

Regulatory Treatment - The OCC, KCC and regulatory authorities in Texas have approved the recovery of pension costs and other postemployment benefits costs through rates for Oklahoma Natural Gas, Kansas Gas Service and Texas Gas Service, respectively. The costs recovered through rates are based on current funding requirements and the net periodic benefit cost for defined benefit pension and other postemployment costs. Differences, if any, between the expense and the amount recovered through rates would be reflected in earnings, net of authorized deferrals.

We historically have recovered defined benefit pension and other postemployment benefit costs through rates. We believe it is probable that regulators will continue to include the net periodic pension and other postemployment benefit costs in our cost of service.

Obligations and Funded Status - The following table sets forth our defined benefit pension and other postemployment benefit plans, benefit obligations and fair value of plan assets for the periods indicated:

 
Pension Benefits
 
Other Postemployment Benefits
 
December 31,
 
December 31,
 
2016
 
2015
 
2016
 
2015
Changes in Benefit Obligation
(Thousands of dollars)
 
 
Benefit obligation, beginning of period
$
985,624

 
$
1,028,171

 
$
228,253

 
$
257,688

Service cost
12,055

 
13,660

 
2,675

 
3,257

Interest cost
45,550

 
43,542

 
10,235

 
10,628

Plan participants’ contributions
—

 
—

 
3,043

 
2,915

Actuarial loss (gain)
25,886

 
(47,607
)
 
14,309

 
(19,702
)
Benefits paid
(71,066
)
 
(52,142
)
 
(15,450
)
 
(14,632
)
Plan amendment
—

 
—

 
483

 
(11,901
)
Settlements
(31,518
)
 
—

 
—

 
—

   Benefit obligation, end of period
966,531

 
985,624

 
243,548

 
228,253

 
 
 
 
 
 
 
 
Change in Plan Assets
 
 
 
 
 
 
 
Fair value of plan assets, beginning of period
785,161

 
845,396

 
155,495

 
151,777

Actual return on plan assets
48,768

 
(9,026
)
 
9,733

 
1,335

Employer contributions
12,441

 
933

 
13,225

 
14,100

Plan participants’ contributions
—

 
—

 
3,043

 
2,915

Benefits paid
(71,066
)
 
(52,142
)
 
(15,450
)
 
(14,632
)
Settlements
(35,718
)
 
—

 
—

 
—

   Fair value of assets, end of period
739,586

 
785,161

 
166,046

 
155,495

   Balance at December 31
$
(226,945
)
 
$
(200,463
)
 
$
(77,502
)
 
$
(72,758
)
 
 
 
 
 
 
 
 
Current liabilities
$
(941
)
 
$
(912
)
 
$
—

 
$
—

Noncurrent liabilities
(226,004
)
 
(199,551
)
 
(77,502
)
 
(72,758
)
   Balance at December 31
$
(226,945
)
 
$
(200,463
)
 
$
(77,502
)
 
$
(72,758
)


In the fourth quarter of 2016, we settled a portion of our benefit obligation with the purchase of annuities. Benefits paid reflects $18.1 million of lump sum payments to certain terminated vested participants. The accumulated benefit obligation for our defined benefit pension plans was $912.4 million and $934.3 million at December 31, 2016 and 2015, respectively.

There are no plan assets expected to be withdrawn and returned to us in 2017.

Components of Net Periodic Benefit Cost - The following tables set forth the components of net periodic benefit cost for our defined benefit pension and other postemployment benefit plans for the period indicated:

 
Pension Benefits
 
Year Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
 
 
 
Service cost
$
12,055

 
$
13,660

 
$
11,620

Interest cost
45,550

 
43,542

 
43,791

Expected return on assets
(61,183
)
 
(61,769
)
 
(59,862
)
Amortization of unrecognized prior service cost
—

 
266

 
549

Amortization of net loss
35,543

 
42,226

 
30,200

Settlements
—

 
27

 
773

   Net periodic benefit cost
$
31,965

 
$
37,952

 
$
27,071


 
Other Postemployment Benefits
 
Year Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
 
 
 
Service cost
$
2,675

 
$
3,257

 
$
3,468

Interest cost
10,235

 
10,628

 
11,605

Expected return on assets
(12,370
)
 
(11,892
)
 
(11,393
)
Amortization of unrecognized prior service cost
(3,316
)
 
(2,228
)
 
(1,760
)
Amortization of net loss
5,369

 
5,268

 
3,969

   Net periodic benefit cost
$
2,593

 
$
5,033

 
$
5,889



Other Comprehensive Income (Loss) - The following table sets forth the amounts recognized in other comprehensive income (loss) related to our defined benefit pension benefits for the period indicated:

 
Pension Benefits
 
Year Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Net gain (loss) arising during the period
$
(1,262
)
 
$
339

 
$
(3,543
)
Amortization of loss
751

 
917

 
518

Deferred income taxes
197

 
(483
)
 
1,244

   Total recognized in other comprehensive income (loss)
$
(314
)
 
$
773

 
$
(1,781
)


There were no amounts recognized in other comprehensive income (loss) related to our other postemployment benefits for the periods presented.

The tables below set forth the amounts in accumulated other comprehensive income (loss) that had not yet been recognized as components of net periodic benefit expense for the periods indicated:

 
Pension Benefits
 
December 31,
 
2016
 
2015
 
(Thousands of dollars)
Prior service credit (cost)
$
—

 
$
—

Accumulated loss
(414,757
)
 
(407,798
)
Accumulated other comprehensive loss
  before regulatory assets
(414,757
)
 
(407,798
)
Regulatory asset for regulated entities
407,073

 
400,625

Accumulated other comprehensive loss
  after regulatory assets
(7,684
)
 
(7,173
)
Deferred income taxes
2,969

 
2,772

Accumulated other comprehensive loss,
  net of tax
$
(4,715
)
 
$
(4,401
)

 
Other Postemployment Benefits
 
December 31,
 
2016
 
2015
 
(Thousands of dollars)
Prior service credit (cost)
$
10,211

 
$
14,010

Accumulated loss
(62,084
)
 
(50,447
)
Accumulated other comprehensive loss
  before regulatory assets
(51,873
)
 
(36,437
)
Regulatory asset for regulated entities
51,873

 
36,437

Accumulated other comprehensive loss
  after regulatory assets
—

 
—

Deferred income taxes
—

 
—

Accumulated other comprehensive loss,
  net of tax
$
—

 
$
—



The following table sets forth the amounts recognized in either accumulated comprehensive income (loss) or regulatory assets expected to be recognized as components of net periodic benefit expense in the next fiscal year:

 
Pension Benefits
 
Other Postemployment Benefits
Amounts to be recognized in 2017
(Thousands of dollars)
Prior service credit (cost)
$
—

 
$
(4,597
)
Actuarial net loss
$
36,107

 
$
6,484



Health Care Cost Trend Rates - The following table sets forth the assumed health care cost-trend rates for the periods indicated:


2016
 
2015
Health care cost-trend rate assumed for next year
7.25%
 
4.00% - 7.50%
Rate to which the cost-trend rate is assumed to decline
  (the ultimate trend rate)
5.00%
 
4.00% - 5.00%
Year that the rate reaches the ultimate trend rate
2022
 
2022


Assumed health care cost-trend rates have a significant effect on the amounts reported for our health care plans. A one percentage point change in assumed health care cost-trend rates would have the following effects:


One Percentage

One Percentage

Point Increase

Point Decrease

(Thousands of dollars)
Effect on total of service and interest cost
$
233


$
(232
)
Effect on other postemployment benefit obligation
$
3,937


$
(3,991
)


Plan Assets - Our investment strategy is to invest plan assets in accordance with sound investment practices that emphasize long-term fundamentals. The goal of this strategy is to maximize investment returns while managing risk in order to meet the plan’s current and projected financial obligations. To achieve this strategy, we have established a liability-driven investment strategy to change the allocations as the plan reaches certain funded status. The plan’s investments include a diverse blend of various domestic and international equities, investment-grade debt securities which mirror the cash flows of our liability, insurance contracts and alternative investments. The current target allocation for the assets of our defined benefit pension plan is as follows:
 
 
U.S. large-cap equities
37.4
%
Investment-grade bonds
30.0
%
Developed foreign large-cap equities
10.6
%
Alternative investments
7.7
%
Mid-cap equities
5.6
%
Emerging markets equities
5.0
%
Small-cap equities
3.7
%
  Total
100
%

As part of our risk management for the plans, minimums and maximums have been set for each of the asset classes listed above. All investment managers for the plan are subject to certain restrictions on the securities they purchase and, with the exception of indexing purposes, are prohibited from owning our stock.

The current target allocation for the assets of our other postemployment benefits plan is 30 percent fixed income securities and 70 percent equity securities.

The following tables set forth our pension benefits and other postemployment benefits plan assets by fair value category as of the measurement date:


Pension Benefits

December 31, 2016
Asset Category
Level 1
Level 2
Level 3
Total

(Thousands of dollars)
Investments:




Equity securities (a)
$
371,655

$
58,987

$
—

$
430,642

Government obligations
—

47,445

—

47,445

Corporate obligations (b)
—

129,036

—

129,036

Cash and money market funds (c)
13,786

16,114

—

29,900

Insurance contracts and group annuity contracts
—

—

45,140

45,140

Other investments (d)
—

71

57,352

57,423

  Total assets
$
385,441

$
251,653

$
102,492

$
739,586

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.
(d) - This category represents alternative investments such as hedge funds and other financial instruments.

 
Pension Benefits
 
December 31, 2015
Asset Category
Level 1
Level 2
Level 3
Total
 
(Thousands of dollars)
Investments:
 
 
 
 
Equity securities (a)
$
405,935

$
62,150

$
—

$
468,085

Government obligations
—

44,651

—

44,651

Corporate obligations (b)
—

139,396

—

139,396

Cash and money market funds (c)
5,429

10,279

—

15,708

Insurance contracts and group annuity contracts
—

—

56,465

56,465

Other investments (d)
2,884

—

57,972

60,856

  Total assets
$
414,248

$
256,476

$
114,437

$
785,161

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.
(d) - This category represents alternative investments such as hedge funds and other financial instruments.

 
Other Postemployment Benefits
 
December 31, 2016
Asset Category
Level 1
Level 2
Level 3
Total
 
(Thousands of dollars)
Investments:
 
 
 
 
Equity securities (a)
$
39,817

$
7,323

$
—

$
47,140

Government obligations
—

75

—

75

Corporate obligations (b)
—

19,948

—

19,948

Cash and money market funds (c)
74

16,989

—

17,063

Insurance contracts and group annuity contracts
—

81,820

—

81,820

  Total assets
$
39,891

$
126,155

$
—

$
166,046

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.


 
Other Postemployment Benefits
 
December 31, 2015
Asset Category
Level 1
Level 2
Level 3
Total
 
(Thousands of dollars)
Investments:
 
 
 
 
Equity securities (a)
$
54,560

$
7,498

$
—

$
62,058

Government obligations
—

64

—

64

Corporate obligations (b)
—

200

—

200

Cash and money market funds (c)
233

13,322

—

13,555

Insurance contracts and group annuity contracts
—

79,531

—

79,531

Other investments (d)
4

—

83

87

  Total assets
$
54,797

$
100,615

$
83

$
155,495

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.
(d) - This category represents alternative investments such as hedge funds.

The following table sets forth the reconciliation of Level 3 fair value measurements of our pension plans for the periods indicated:

 
Pension Benefits
 
Insurance
Contracts
 
Other
Investments
 
Total
 
(Thousands of dollars)
January 1, 2015
$
59,877

 
$
57,914

 
$
117,791

Net realized and unrealized gains (losses)
2,188

 
58

 
2,246

Settlements
(5,600
)
 
—

 
(5,600
)
December 31, 2015
$
56,465

 
$
57,972

 
$
114,437

Net realized and unrealized gains (losses)
4,518

 
(620
)
 
3,898

Sales and settlements
(15,843
)
 
—

 
(15,843
)
December 31, 2016
$
45,140

 
$
57,352

 
$
102,492



Contributions - During 2016, we contributed $12.4 million to our defined benefit pension plans and we contributed $13.2 million to our other postemployment benefit plans. In 2017, we expect to contribute $1.0 million to our defined benefit pension plans and expect to contribute $3.1 million to our other postemployment benefit plans.

Pension and Other Postemployment Benefit Payments - Benefit payments for our defined benefit pension and other postemployment benefit plans for the period ended December 31, 2016 were $71.1 million and $15.5 million, respectively. The following table sets forth the pension benefits and other postemployment benefits payments expected to be paid in 2017-2026:

 
Pension
Benefits
 
Other Postemployment
Benefits
Benefits to be paid in:
(Thousands of dollars)
2017
$
51,539

 
$
16,165

2018
$
52,660

 
$
16,815

2019
$
53,450

 
$
17,073

2020
$
54,812

 
$
17,379

2021
$
56,033

 
$
17,401

2022 through 2026
$
294,519

 
$
86,559



The expected benefits to be paid are based on the same assumptions used to measure our benefit obligation at December 31, 2016, and include estimated future employee service.

Other Employee Benefit Plans

401(k) Plan - We have a 401(k) Plan which covers all full-time employees, and employee contributions are discretionary. We match 100 percent of each participant’s eligible contribution up to 6 percent of eligible compensation, subject to certain limits. Our contributions made to the plan were $10.8 million, $10.2 million and $9.7 million in 2016, 2015 and 2014, respectively.

Profit-Sharing Plan - We have a profit-sharing plan for all employees that do not participate in our defined benefit pension plan. We plan to make a contribution to the profit-sharing plan each quarter equal to 1 percent of each participant’s eligible compensation during the quarter. Additional discretionary employer contributions may be made at the end of each year. Employee contributions are not allowed under the plan. Our contributions made to the plan were $6.0 million, $6.5 million and $4.0 million in 2016, 2015 and 2014, respectively.

Employee Deferred Compensation Plan - Our Nonqualified Deferred Compensation Plan provides select employees with the option to defer portions of their compensation and provides nonqualified deferred compensation benefits that are not available due to limitations on employer and employee contributions to qualified defined contribution plans under the federal tax laws. Contributions made to the plan were not material in 2016, 2015 and 2014.
INCOME TAXES (Notes)
INCOME TAXES
12.
INCOME TAXES

The following table sets forth our provision for income taxes for the periods indicated:

 
Years Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Current income tax provision
 
 
 
 
 
Federal
$
(2,016
)
 
$
7,135

 
$
17,006

State
471

 
2,055

 
1,397

Total current income tax provision
(1,545
)
 
9,190

 
18,403

Deferred income tax provision
 
 
 
 
 
Federal
76,247

 
56,440

 
42,024

State
10,541

 
7,349

 
7,911

Total deferred income tax provision
86,788

 
63,789

 
49,935

Total provision for income taxes
$
85,243

 
$
72,979

 
$
68,338



The following table is a reconciliation of our income tax provision for the periods indicated:
 
Years Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Income before income taxes
$
225,338

 
$
192,009

 
$
178,128

Federal statutory income tax rate
35
%
 
35
%
 
35
%
Provision for federal income taxes
78,868

 
67,203

 
62,345

State income taxes, net of federal tax benefit
7,158

 
6,114

 
6,051

Other, net
(783
)
 
(338
)
 
(58
)
Total provision for income taxes
$
85,243

 
$
72,979

 
$
68,338



The following table sets forth the tax effects of temporary differences that gave rise to significant portions of the deferred tax assets and liabilities for the periods indicated:
 
December 31,
 
2016
 
2015
 
(Thousands of dollars)
Deferred tax assets
 
 
 
Employee benefits and other accrued liabilities
$
123,333

 
$
110,148

Net operating loss
23,094

 
—

Other
5,716

 
7,848

Total deferred tax assets
152,143

 
117,996

Deferred tax liabilities
 
 
 
Excess of tax over book depreciation
990,682

 
897,667

Purchased-gas cost adjustment
13,822

 
3,999

Other regulatory assets and liabilities, net
186,207

 
168,115

Total deferred tax liabilities
1,190,711

 
1,069,781

Net deferred tax liabilities
$
1,038,568

 
$
951,785



As of December 31, 2016, we have federal and state income tax net operating loss (NOL) carryforwards of $63.0 million and $21.0 million, respectively, which will expire at various dates from 2024 through 2036. We believe that it is more likely than not that the tax benefits of the NOL carryforwards will be utilized prior to their expirations; therefore, no valuation allowance is necessary.

Deferred tax assets related to tax benefits of employee share-based compensation have been reduced for performance share units and restricted share units that vested in periods in which we were in an NOL position. This vesting resulted in tax deductions in excess of previously recorded benefits based on the performance share unit and restricted share unit value at the time of grant. Although these additional tax benefits are reflected in NOL carryforwards in the tax return, the additional tax
benefit is not recognized until the deduction reduces taxes payable. A portion of the tax benefit does not reduce our current taxes payable due to NOL carryforwards; accordingly, these tax benefits are not reflected in our NOLs in deferred tax assets. Cumulative tax benefits included in NOL carryforwards but not reflected in deferred tax assets were $11.0 million as of December 31, 2016.

We have filed our consolidated federal and state tax returns for years 2014 and 2015.
COMMITMENTS AND CONTINGENCIES (Notes)
COMMITMENTS AND CONTINGENCIES
13.
COMMITMENTS AND CONTINGENCIES

Commitments - Operating leases represent future minimum lease payments under noncancelable leases covering office space, facilities and information technology hardware and software. Rental expense was $8.6 million in 2016 and $5.0 million in each of 2015 and 2014. The following table sets forth our operating lease payments for the periods indicated:
Operating Leases
(Millions of dollars)
2017
 
$
5.6

2018
 
5.2

2019
 
4.4

2020
 
3.6

2021
 
3.2

Thereafter
 
4.4

Total
 
$
26.4



Environmental Matters - We are subject to multiple historical, wildlife preservation and environmental laws and/or regulations, which affect many aspects of our present and future operations. Regulated activities include, but are not limited to, those involving air emissions, storm water and wastewater discharges, handling and disposal of solid and hazardous wastes, wetland preservation, hazardous materials transportation, and pipeline and facility construction. These laws and regulations require us to obtain and/or comply with a wide variety of environmental clearances, registrations, licenses, permits and other approvals. Failure to comply with these laws, regulations, licenses and permits may expose us to fines, penalties and/or interruptions in our operations that could be material to our results of operations. In addition, emission controls and/or other regulatory or permitting mandates under the Clean Air Act and other similar federal and state laws could require unexpected capital expenditures. We cannot assure that existing environmental statutes and regulations will not be revised or that new regulations will not be adopted or become applicable to us. Revised or additional statutes or regulations that result in increased compliance costs or additional operating restrictions could have a material adverse effect on our business, financial condition, results of operations and cash flows.

We own or retain legal responsibility for the environmental conditions at 12 former manufactured natural gas sites in Kansas. These sites contain potentially harmful materials that are subject to control or remediation under various environmental laws and regulations. A consent agreement with the KDHE governs all work at these sites. The terms of the consent agreement require us to investigate these sites and set remediation activities based upon the results of the investigations and risk analysis. Remediation typically involves the management of contaminated soils and may involve removal of structures and monitoring and/or remediation of groundwater.

We have completed or addressed removal of the source of soil contamination at 11 of the 12 sites, and continue to monitor groundwater at eight of the 12 sites according to plans approved by the KDHE. Regulatory closure has been achieved at three of the sites, subject to any future regulatory remediation requirements that may require additional costs. During 2016, we completed a site assessment at the twelfth site where no active soil remediation has occurred. We have submitted a work plan to the KDHE for approval to remove contaminated soil at this site. Costs associated with the remediation at this site are not expected to be material to our results of operations or financial position.

With regard to one of our other former manufactured natural gas sites, recent results from periodic monitoring and a 2016 interim site investigation indicated elevated levels of potentially harmful materials at the site. In response to the results of the interim site investigation, during the fourth quarter of 2016, potential investigation and remediation alternatives were developed. We have estimated the potential costs associated with additional investigation and remediation to be in the range of $4.0 million to $7.0 million. Additional testing and work plan development will be conducted in 2017 to determine a remediation work plan to present to the KDHE for approval and could impact our estimates of the cost of remediation at this site. A single reliable estimate of the remediation costs is not feasible due to the amount of uncertainty in the ultimate remediation approach that will be utilized. Accordingly, in the fourth quarter of 2016, we recorded a reserve of $4.0 million for this site.

Our expenditures for environmental evaluation, mitigation, remediation and compliance to date have not been significant in relation to our financial position, results of operations or cash flows, and our expenditures related to environmental matters had no material effects on earnings or cash flows during 2016, 2015 and 2014. A number of environmental issues may exist with respect to manufactured gas plants that are unknown to us. Accordingly, future costs are dependent on the final determination and regulatory approval of any remedial actions, the complexity of the site, level of remediation required, changing technology and governmental regulations, and to the extent not recovered by insurance or recoverable in rates from our customers, could be material to our financial condition, results of operations or cash flows.

With the trend toward stricter standards, greater regulation and more extensive permit requirements for the types of assets operated by us that are subject to environmental regulation, our environmental expenditures could increase in the future, and such expenditures may not be fully recovered by insurance or recoverable in rates from our customers, and those costs may adversely affect our financial condition, results of operations and cash flows. We do not expect expenditures for these matters to have a material adverse effect on our financial condition, results of operations or cash flows.

Pipeline Safety - We are subject to PHMSA regulations, including integrity-management regulations. PHMSA regulations require pipeline companies operating high-pressure transmission pipelines to perform integrity assessments on pipeline segments that pass through densely populated areas or near specifically designated high-consequence areas. In January 2012, the Pipeline Safety, Regulatory Certainty and Job Creation Act was signed into law. The law increased maximum penalties for violating federal pipeline safety regulations and directs the DOT and the Secretary of Transportation to conduct further review or studies on issues that may or may not be material to us. These issues include, but are not limited to, the following:
•
an evaluation of whether natural gas pipeline integrity-management requirements should be expanded beyond current high-consequence areas;
•
a verification of records for pipelines in class 3 and 4 locations and high-consequence areas to confirm maximum allowable operating pressures; and
•
a requirement to test previously untested pipelines operating above 30 percent yield strength in high-consequence areas.

In April 2016, PHMSA published a NPRM, the Safety of Gas Transmission & Gathering Lines Rule, in the Federal Register to revise pipeline safety regulations applicable to the safety of onshore natural gas transmission and gathering pipelines. Proposals include changes to pipeline integrity management requirements and other safety-related requirements. The NPRM comment period ended July 7, 2016, and comments are under review by PHMSA. The potential capital and operating expenditures associated with the NPRM are currently being evaluated and could be significant depending on the final regulations.

Legal Proceedings - We are a party to various litigation matters and claims that have arisen in the normal course of our operations. While the results of litigation and claims cannot be predicted with certainty, we believe the reasonably possible losses from such matters, individually and in the aggregate, are not material. Additionally, we believe the probable final outcome of such matters will not have a material adverse effect on our results of operations, financial position or cash flows.
QUARTERLY FINANCIAL DATA (UNAUDITED) (Notes)
QUARTERLY FINANCIAL DATA (UNAUDITED)
14.
QUARTERLY FINANCIAL DATA (UNAUDITED)

 
 
First
Quarter
 
Second
Quarter
 
Third
Quarter
 
Fourth
Quarter
Year Ended December 31, 2016
 
 
 
 
 
 
(Thousands of dollars)
Revenues
 
$
508,364

 
$
245,923

 
$
232,191

 
$
440,754

Operating income
 
$
116,073

 
$
43,621

 
$
30,892

 
$
78,534

Net income
 
$
64,743

 
$
20,300

 
$
12,737

 
$
42,315

Earnings per share
 
 
 
 
 
 
 
 
   Basic
 
$
1.23

 
$
0.39

 
$
0.24

 
$
0.81

   Diluted
 
$
1.22

 
$
0.38

 
$
0.24

 
$
0.80

 
 
First
Quarter
 
Second
Quarter
 
Third
Quarter
 
Fourth
Quarter
Year Ended December 31, 2015
 
 
 
 
 
 
(Thousands of dollars)
Revenues
 
$
676,531

 
$
256,786

 
$
225,226

 
$
389,149

Operating income
 
$
109,005

 
$
31,270

 
$
24,951

 
$
73,903

Net income
 
$
60,381

 
$
12,076

 
$
7,371

 
$
39,202

Earnings per share
 
 
 
 
 
 
 
 
   Basic
 
$
1.15

 
$
0.23

 
$
0.14

 
$
0.75

   Diluted
 
$
1.13

 
$
0.23

 
$
0.14

 
$
0.74

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)
Basis of Accounting 
Basis of Presentation - Prior to our separation from ONEOK, our financial statements were derived from ONEOK’s financial statements, which included its natural gas distribution business as if we, for accounting purposes, had been a separate company for all periods presented. The assets and liabilities in the financial statements have been reflected on a historical basis. The financial statements for the period prior to the separation also includes expense allocations for certain corporate functions historically performed by ONEOK, including allocations of general corporate expenses related to executive oversight, accounting, treasury, tax, legal, information technology and other services. We believe our assumptions underlying the financial statements, including the assumptions regarding the allocation of general corporate expenses from ONEOK, are reasonable. However, the financial statements may not include all of the actual expenses that would have been incurred by us and may not reflect our results of operations, financial position and cash flows had we been a separate publicly traded company during the period presented prior to the separation.

All financial information presented after the separation represents the results of operations, financial position and cash flows of ONE Gas. Accordingly:
•
Our Statements of Income and Comprehensive Income for the year ended December 31, 2014, consist of the results of ONE Gas for the eleven months ended December 31, 2014, and the results of ONE Gas Predecessor for the one month ended January 31, 2014.
•
Our Statement of Cash Flows for the year ended December 31, 2014, consists of the results of ONE Gas for the eleven months ended December 31, 2014, and the results of ONE Gas Predecessor for the one month ended January 31, 2014.
•
Our Statement of Equity for the year ended December 31, 2014, consists of both the activity for ONE Gas Predecessor prior to January 31, 2014, and the activity for ONE Gas completed in connection with, and subsequent to, the separation on January 31, 2014.

The financial statements include the accounts of the natural gas distribution business as set forth in “Organization and Nature of Operations” above. All significant balances and transactions between our divisions have been eliminated.
Use of Estimates 
Use of Estimates - The preparation of our financial statements and related disclosures in accordance with GAAP requires us to make estimates and assumptions with respect to values or conditions that cannot be known with certainty that affect the reported amount of assets and liabilities, and the disclosure of contingent assets and liabilities at the date of the financial statements. These estimates and assumptions also affect the reported amounts of revenue and expenses during the reporting period. Items that may be estimated include, but are not limited to, the economic useful life of assets, fair value of assets and liabilities, provisions for doubtful accounts receivable, unbilled revenues for natural gas delivered but for which meters have not been read, natural gas purchased but for which no invoice has been received, provision for income taxes, including any deferred tax valuation allowances, the results of litigation and various other recorded or disclosed amounts.

We evaluate these estimates on an ongoing basis using historical experience and other methods we consider reasonable based on the particular circumstances. Nevertheless, actual results may differ significantly from the estimates. Any effects on our financial position or results of operations from revisions to these estimates are recorded in the period when the facts that give rise to the revision become known.
Fair Value Measurement 
Fair Value Measurements - We define fair value as the price that would be received from the sale of an asset or the transfer of a liability in an orderly transaction between market participants at the measurement date. We use the market and income approaches to determine the fair value of our assets and liabilities and consider the markets in which the transactions are executed. We measure the fair value of a group of financial assets and liabilities consistent with how a market participant would price the net risk exposure at the measurement date.

Fair Value Hierarchy - At each balance sheet date, we utilize a fair value hierarchy to classify fair value amounts recognized or disclosed in our financial statements based on the observability of inputs used to estimate such fair value. The levels of the hierarchy are described below:
•
Level 1 - Unadjusted quoted prices in active markets for identical assets or liabilities;
•
Level 2 - Significant observable pricing inputs other than quoted prices included within Level 1 that are, either directly or indirectly, observable as of the reporting date. Essentially, this represents inputs that are derived principally from or corroborated by observable market data; and
•
Level 3 - May include one or more unobservable inputs that are significant in establishing a fair value estimate. These unobservable inputs are developed based on the best information available and may include our own internal data.

We recognize transfers into and out of the levels as of the end of each reporting period.

Determining the appropriate classification of our fair value measurements within the fair value hierarchy requires management’s judgment regarding the degree to which market data is observable or corroborated by observable market data. We categorize derivatives for which fair value is determined using multiple inputs within a single level, based on the lowest level input that is significant to the fair value measurement in its entirety. See Note 7 for additional information regarding our fair value measurements.
Cash and Cash Equivalents 
Cash and Cash Equivalents - Cash equivalents consist of highly liquid investments, which are readily convertible into cash and have original maturities of three months or less.
Revenue Recognition 
Revenue Recognition - For regulated deliveries of natural gas, we read meters and bill customers on a monthly cycle. We recognize revenues upon the delivery of the natural gas commodity or services rendered to customers. The billing cycles for customers do not necessarily coincide with the accounting periods used for financial reporting purposes. Revenues are accrued for natural gas delivered and services rendered to customers, but not yet billed. Accrued unbilled revenue is based on a percentage estimate of amounts unbilled each month, which is dependent upon a number of factors, some of which require management’s judgment. These factors include customer consumption patterns and the impact of weather on usage. The amounts of accrued unbilled natural gas sales revenues at December 31, 2016 and 2015, were $143.2 million and $109.6 million, respectively.
Accounts Receivable 
Accounts Receivable - Accounts receivable represent valid claims against nonaffiliated customers for natural gas sold or services rendered, net of allowances for doubtful accounts. We assess the creditworthiness of our customers. Those customers who do not meet minimum standards are required to provide security, including deposits and other forms of collateral, when appropriate. With more than 2 million customers across three states, we are not exposed materially to a concentration of credit risk. We maintain an allowance for doubtful accounts based upon factors surrounding the credit risk of customers, historical trends, consideration of the current credit environment and other information. In Oklahoma, Kansas and most jurisdictions we serve in Texas, we are able to recover natural gas costs related to doubtful accounts through purchased-gas cost adjustment mechanisms. At December 31, 2016 and 2015, our allowance for doubtful accounts was $4.2 million and $3.5 million, respectively.
Inventories 
Inventories - Natural gas in storage is maintained on the basis of weighted-average cost. Natural gas inventories that are injected into storage are recorded in inventory based on actual purchase costs, including storage and transportation costs. Natural gas inventories that are withdrawn from storage are accounted for in our purchased-gas cost adjustment mechanisms at the weighted-average inventory cost.

Materials and supplies inventories are stated at the lower of weighted-average cost or net realizable value.
Derivatives and Risk Management Activities 
Derivatives and Risk Management Activities - We record all derivative instruments at fair value, with the exception of normal purchases and normal sales that are expected to result in physical delivery. The accounting for changes in the fair value of a derivative instrument depends on whether it has been designated and qualifies as part of a hedging relationship and, if so, the reason for holding it, or if regulatory rulings require a different accounting treatment.

If certain conditions are met, we may elect to designate a derivative instrument as a hedge of exposure to changes in fair values or cash flows.

The table below summarizes the various ways in which we account for our derivative instruments and the impact on our financial statements:
 
 
Recognition and Measurement
Accounting Treatment
 
Balance Sheet
 
Income Statement
Normal purchases and
normal sales
-
Fair value not recorded
-
Change in fair value not recognized in earnings
Mark-to-market
-
Recorded at fair value
-
Change in fair value recognized in, and
recoverable through, the purchased-gas cost adjustment mechanisms

We have not elected to formally designate any of our derivative instruments as hedges. Gains or losses associated with the fair value of commodity derivative instruments entered into by us are included in, and recoverable through, the purchased-gas cost adjustment mechanisms.

See Note 7 for additional information regarding our fair value measurements and hedging activities using derivatives.
Property, Plant and Equipment 
Property, Plant and Equipment - Our properties are stated at cost, which includes direct construction costs such as direct labor, materials, burden and AFUDC. Generally, the cost of our property retired or sold, plus removal costs, less salvage, is charged to accumulated depreciation. Gains and losses from sales or retirement of an entire operating unit or system of our properties are recognized in income. Maintenance and repairs are charged directly to expense.

AFUDC represents the cost of borrowed funds used to finance construction activities. We capitalize interest costs during the construction or upgrade of qualifying assets. Capitalized interest is recorded as a reduction to interest expense.

Our properties are depreciated using the straight-line method over their estimated useful lives. Generally, we apply composite depreciation rates to functional groups of property having similar economic circumstances. We periodically conduct depreciation studies to assess the economic lives of our assets. These depreciation studies are completed as a part of our regulatory proceedings, and the changes in economic lives, if applicable, are implemented prospectively when the new rates are effective. Changes in the estimated economic lives of our property, plant and equipment could have a material effect on our financial position, results of operations or cash flows.

Property, plant and equipment on our Balance Sheets includes construction work in process for capital projects that have not yet been placed in service and therefore are not being depreciated. Assets are transferred out of construction work in process when they are substantially complete and ready for their intended use.

See Note 9 for additional information regarding our property, plant and equipment.

Impairment of Goodwill and Long-Lived Assets 
Impairment of Goodwill and Long-Lived Assets - We assess our goodwill for impairment at least annually as of July 1. Our goodwill impairment analysis performed in 2016, 2015 and 2014, utilized a qualitative assessment and did not result in any impairment indicators. Subsequent to July 1, 2016, no event has occurred indicating that it is more likely than not that our fair value is less than our carrying value of our net assets.

As part of our goodwill impairment test, we first assess qualitative factors (including macroeconomic conditions, industry and market considerations, cost factors and overall financial performance) to determine whether it is more likely than not that our fair value is less than our carrying amount. If further testing is necessary, we perform a two-step impairment test for goodwill. In the first step, an initial assessment is made by comparing our fair value with our book value, including goodwill. If the fair value is less than the book value, an impairment is indicated, and we must perform a second test to measure the amount of the impairment. In the second test, we calculate the implied fair value of the goodwill by deducting the fair value of all tangible and intangible net assets from the fair value determined in step one of the assessment. If the carrying value of the goodwill exceeds the implied fair value of the goodwill, we will record an impairment charge.

To estimate our fair value, we use two generally accepted valuation approaches, an income approach and a market approach, using assumptions consistent with a market participant’s perspective. Under the income approach, we use anticipated cash flows over a period of years plus a terminal value and discount these amounts to their present value using appropriate discount rates. Under the market approach, we apply acquisition multiples to forecasted cash flows. The acquisition multiples used are consistent with historical market transactions. The forecasted cash flows are based on average forecasted cash flows over a period of years.

We assess our long-lived assets for impairment whenever events or changes in circumstances indicate that an asset’s carrying amount may not be recoverable. An impairment is indicated if the carrying amount of a long-lived asset exceeds the sum of the undiscounted future cash flows expected to result from the use and eventual disposition of the asset. If an impairment is indicated, we record an impairment loss equal to the difference between the carrying value and the fair value of the long-lived asset. We determined that there were no asset impairments in 2016, 2015 or 2014.
Regulation 
Regulation - We are subject to the rate regulation and accounting requirements of the OCC, KCC, RRC and various municipalities in Texas. We follow the accounting and reporting guidance for regulated operations. During the ratemaking process, regulatory authorities set the framework for what we can charge customers for our services and establish the manner that our costs are accounted for, including allowing us to defer recognition of certain costs and permitting recovery of the amounts through rates over time, as opposed to expensing such costs as incurred. Examples include weather normalization, unrecovered purchased-gas costs, pension and postemployment benefit costs and ad-valorem taxes. This allows us to stabilize rates over time rather than passing such costs on to the customer for immediate recovery. Actions by regulatory authorities could have an effect on the amount recovered from rate payers. Any difference in the amount recoverable and the amount deferred is recorded as income or expense at the time of the regulatory action. A write-off of regulatory assets and costs not recovered may be required if all or a portion of the regulated operations have rates that are no longer:
•
established by independent regulators;
•
designed to recover the specific entity’s costs of providing regulated services; and
•
set at levels that will recover our costs when considering the demand and competition for our services.

See Note 8 for additional information regarding our regulatory assets and liabilities disclosures.
Pension and Other Postretirement Plans 
Pension and Other Postemployment Employee Benefits - We have defined benefit retirement plans covering eligible employees. We also sponsor welfare plans that provide other postemployment medical and life insurance benefits to eligible employees who retire with at least five years of service. To calculate the costs and liabilities related to our plans, we utilize an outside actuarial consultant, which uses statistical and other factors to anticipate future events. These factors include assumptions about the discount rate, expected return on plan assets, rate of future compensation increases, age and mortality and employment periods. In determining the projected benefit obligations and costs, assumptions can change from period to period and may result in material changes in the cost and liabilities we recognize.

Income Taxes 
Income Taxes - Deferred income taxes are recorded for the difference between the financial statement and income tax basis of assets and liabilities and carryforward items, based on income tax laws and rates existing at the time the temporary differences are expected to reverse. The effect on deferred taxes of a change in tax rates is deferred and amortized for operations regulated by the OCC, KCC, RRC and various municipalities in Texas, if, as a result of an action by a regulator, it is probable that the effect of the change in tax rates will be recovered from or returned to customers through future rates. We continue to amortize previously deferred investment tax credits for ratemaking purposes over the periods prescribed by our regulators.

A valuation allowance for deferred tax assets is recognized when it is more likely than not that some or all of the benefit from the deferred tax asset will not be realized. To assess that likelihood, we use estimates and judgment regarding our future taxable income, as well as the jurisdiction in which such taxable income is generated, to determine whether a valuation allowance is required. Such evidence can include our current financial position, our results of operations, both actual and forecasted, the reversal of deferred tax liabilities, as well as the current and forecasted business economics of our industry. We had no valuation allowance at December 31, 2016 and 2015.

We utilize a more-likely-than-not recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position that is taken or expected to be taken in a tax return. We reflect penalties and interest as part of income tax expense as they become applicable for tax provisions that do not meet the more-likely-than-not recognition threshold and measurement attribute. There were no material uncertain tax positions at December 31, 2016 and 2015. See Note 12 for additional information regarding income taxes.

Asset Retirement Obligations 
Asset Retirement Obligations - Asset retirement obligations represent legal obligations associated with the retirement of long-lived assets that result from the acquisition, construction, development and/or normal use of the asset. Certain long-lived assets that comprise our natural gas distribution systems, primarily our pipeline assets, are subject to agreements or regulations that give rise to an asset retirement obligation for removal or other disposition costs associated with retiring the assets in place upon the discontinued use of the natural gas distribution system. We recognize the fair value of a liability for an asset retirement obligation in the period when it is incurred if a reasonable estimate of the fair value can be made. We are not able to estimate reasonably the fair value of the asset retirement obligations for portions of our assets because the settlement dates are indeterminable given our expected continued use of the assets with proper maintenance. We expect our natural gas distribution systems will continue in operation as long as natural gas supply and demand for natural gas distribution service exists. Based on the widespread use of natural gas for heating and cooking activities by residential and commercial customers in our service areas, management expects supply and demand to exist for the foreseeable future.

In accordance with long-standing regulatory treatment, we collect through rates the estimated costs of removal on certain regulated properties through depreciation expense, with a corresponding credit to accumulated depreciation and amortization. These removal costs collected through our rates include costs attributable to legal and nonlegal removal obligations; however, the amounts collected that are in excess of these nonlegal asset-removal costs incurred are accounted for as a regulatory liability for financial reporting purposes. Historically, with the exception of the regulatory authority in Kansas, the regulatory authorities that have jurisdiction over our regulated operations have not required us to quantify or disclose this amount; rather, these costs are addressed prospectively in depreciation rates and are set in each general rate order. We have made an estimate of our regulatory liability using current rates since the last general rate order in each of our jurisdictions if the removal costs collected have exceeded our removal cost incurred; however, for financial reporting purposes, significant uncertainty exists regarding the future disposition of this regulatory liability, pending, among other issues, clarification of regulatory intent. We continue to monitor the regulatory requirements, and the liability may be adjusted as more information is obtained. We record the estimated asset removal obligation in noncurrent liabilities in other deferred credits on our Balance Sheets. To the extent this estimated liability is adjusted, such amounts will be reclassified between accumulated depreciation and amortization and other deferred credits and therefore will not have an impact on earnings.
Contingencies 
Contingencies - Our accounting for contingencies covers a variety of business activities, including contingencies for legal and environmental exposures. We accrue these contingencies when our assessments indicate that it is probable that a liability has been incurred or an asset will not be recovered and an amount can be estimated reasonably. We expense legal fees as incurred and base our legal liability estimates on currently available facts and our estimates of the ultimate outcome or resolution. Accruals for estimated losses from environmental remediation obligations generally are recognized no later than the completion of a remediation feasibility study. Recoveries of environmental remediation costs from other parties are recorded as assets when their receipt is deemed probable. Actual results may differ from our estimates resulting in an impact, positive or negative, on earnings. See Note 13 for additional information regarding contingencies.
Share-Based Payments 
Share-Based Payments - We expense the fair value of share-based payments net of estimated forfeitures. We estimate forfeiture rates based on historical forfeitures under our share-based payment plans.

Earnings Per Share 
Earnings per share - Basic EPS is based on net income and is calculated based upon the daily weighted-average number of common shares outstanding during the periods presented. Also, this calculation includes fully vested stock awards that have not yet been issued as common stock. Diluted EPS includes the above, plus unvested stock awards granted under our compensation plans, but only to the extent these instruments dilute earnings per share.
Segments 
Segments - We operate in one reportable business segment: regulated public utilities that deliver natural gas to residential, commercial, industrial, wholesale, public authority and transportation customers. We define reportable business segments as components of an organization for which discrete financial information is available and operating results are evaluated on a regular basis by the chief operating decision maker (CODM) in order to assess performance and allocate resources. Our CODM is our Chief Executive Officer (CEO). Characteristics of our organization that were relied upon in making this determination include the similar nature of services we provide, the functional alignment of our organizational structure, and the reports that are regularly reviewed by the CODM for the purpose of assessing performance and allocating resources. Our management is functionally aligned and centralized, with performance evaluated based upon results of the entire distribution business. Capital allocation decisions are driven by asset integrity management, operating efficiency, growth opportunities and government relocations, not geographic location or regulatory jurisdiction.

In 2016, 2015 and 2014, we had no single external customer from which we received 10 percent or more of our gross revenues.
Treasury Stock 
Treasury Stock - We record treasury stock purchases at cost, which includes incremental direct transaction costs. Amounts are recorded as reductions in equity in our Balance Sheets. We record the reissuance of treasury stock at our weighted average cost of treasury shares recorded in equity in our Balance Sheets.
Recently Issues Accounting Standards Updates 
Recently Issued Accounting Standards Update - In January 2017, the FASB issued ASU 2017-04, “Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment,” which simplifies how an entity is required to test goodwill for impairment by eliminating Step 2 of the goodwill test, where the measurement of a goodwill impairment loss was determined by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. Upon adoption, a goodwill impairment will be the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill.  This new guidance is required for our interim and annual reports for periods beginning after December 15, 2019, and early adoption is permitted. We do not expect this guidance to have a material impact on our financial statements and will adjust our goodwill testing procedures accordingly upon adoption.

In March 2016, the FASB issued ASU 2016-09, “Improvements to Employee Share-Based Payment Accounting,” which includes various new aspects to simplify how share-based payments are accounted for and presented in the financial statements. The new standard modifies several aspects of the accounting and reporting for employee share-based payments and related tax accounting impacts, including the presentation in the statements of operations and cash flows. We will adopt this new guidance in the first quarter of 2017. Prospectively, we will record excess tax expenses or benefits in income tax expense. We will record a cumulative-effect increase of $11.0 million to retained earnings, with an offset to a deferred tax asset, as of the beginning of the reporting period in 2017 for excess tax benefits earned prior to January 1, 2017. We will continue our use of the estimation method to account for share unit awards forfeitures rather than actual forfeitures. We will adopt the classification of cash flows for changes in excess tax benefits prospectively in operating activities, and employer withholding shares for tax-withholding purposes for employees retrospectively in investing activities in our statement of cash flows.

In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842),” which prescribes recognizing lease assets and liabilities on the balance sheet and includes disclosure of key information about leasing arrangements.  A modified retrospective transition approach is required for leases existing at the time of adoption. We are evaluating our population of leases, analyzing lease agreements, and holding meetings with cross-divisional teams to determine the potential impact of this accounting standard on our financial position or results of operations and the transition approach we will utilize. This new guidance is required for our interim and annual reports for periods beginning after December 15, 2018, and early adoption is permitted.

In October 2015, the FASB issued ASU 2015-17, “Balance Sheet Classification of Deferred Taxes,” to simplify reporting of deferred taxes. The new guidance requires all deferred tax assets and liabilities, along with any related valuation allowance, be classified as noncurrent on the balance sheet. This guidance is required to be adopted for our interim and annual reports for periods beginning after December 15, 2016, but early adoption is permitted. We have adopted this guidance early to simplify our financial reporting process, have applied it prospectively for the period beginning October 1, 2015, and it did not have a material impact on our financial statements. Prior periods were not retrospectively adjusted.

In August 2015, the FASB issued ASU 2015-15, “Interest-Imputation of Interest (Subtopic 835-30),” which specifically addresses the presentation and subsequent measurement of debt issuance costs associated with line of credit arrangements. We adopted this guidance in the first quarter 2016, and it did not have an impact on our financial position or results of operations.

In April 2015, the FASB issued ASU 2015-03, “Interest-Imputation of Interest,” which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability. We adopted this guidance in the first quarter of 2016, and have applied the changes retrospectively to all periods presented. We have presented such amounts as a direct deduction from the face amount of our long-term debt, rather than in other assets as a deferred charge in our Balance Sheets. Amortization of the debt issuance costs continues to be reported as interest expense in our Statements of Income.

In April 2015, the FASB issued ASU 2015-05, “Intangibles-Goodwill and Other-Internal-Use Software,” which helps entities evaluate the accounting for fees paid by a customer in a cloud computing arrangement. We adopted this guidance prospectively in the first quarter of 2016, and it did not have a material impact on our financial position or results of operations.

In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers,” which clarifies and converges the revenue recognition principles under GAAP and International Financial Reporting Standards. In July 2015, FASB delayed the effective date for one year. We have substantially completed evaluating all of our sources of revenue to determine the potential effect on our financial position, results of operations and cash flows. We continue to monitor accounting task forces and the FASB for additional implementation guidance related to: (1) the accounting for funds received from third parties to partially or fully reimburse the cost of construction of an asset; (2) the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers; and (3) the accounting for alternative revenue programs, such as performance-based ratemaking, that may impact the final conclusions of our evaluation. Until these items are resolved, we cannot determine the effect the new guidance will have on our financial position, results of operations, cash flows, business processes or the transition method we will utilize to adopt the new guidance. We are required to adopt this new guidance for our interim and annual reports beginning with the first quarter 2018.
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Tables)
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] 
The following table sets forth the balance in accumulated other comprehensive income (loss) for the period indicated:
 
 
Accumulated Other Comprehensive Income (Loss)
 
 
(Thousands of dollars)
January 1, 2015
 
$
(5,174
)
Pension and other postemployment benefit plans obligations
 
 
Other comprehensive income (loss) before reclassification, net of tax of $(130)
 
209

Amounts reclassified from accumulated other comprehensive income (loss), net of tax of $(353)
 
564

Other comprehensive income (loss)
 
773

December 31, 2015
 
(4,401
)
Pension and other postemployment benefit plans obligations
 
 
Other comprehensive income (loss) before reclassification, net of tax of $486
 
(776
)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax of $(289)
 
462

Other comprehensive income (loss)
 
(314
)
December 31, 2016
 
$
(4,715
)
Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] 
The following table sets forth the effect of reclassifications from accumulated other comprehensive income (loss) on our Statements of Income for the period indicated:
Details about Accumulated Other Comprehensive Income
 
Year Ended December 31,
Affected Line Item in the
(Loss) Components
 
2016
 
2015
 
2014
Statements of Income
 
 
(Thousands of dollars)
 
Pension and other postemployment benefit plan obligations (a)
 
 
 
 
 
 
 
Amortization of net loss
 
$
40,912

 
$
47,494

 
$
34,169

 
Amortization of unrecognized prior service cost
 
(3,316
)
 
(1,962
)
 
(1,211
)
 
 
 
37,596

 
45,532

 
32,958

 
Regulatory adjustments (b)
 
(36,845
)
 
(44,615
)
 
(32,445
)
 
 
 
751

 
917

 
513

Income before income taxes
 
 
(289
)
 
(353
)
 
(198
)
Income tax expense
Total reclassifications for the period
 
$
462

 
$
564

 
$
315

Net income
(a) These components of accumulated other comprehensive income (loss) are included in the computation of net periodic benefit cost. See Note 11 for additional information regarding our net periodic benefit cost.
(b) Regulatory adjustments represent pension and other postemployment benefit costs expected to be recovered through rates and are deferred as part of our regulatory assets. See Note 8 for additional information regarding our regulatory assets and liabilities.
EARNINGS PER SHARE (Tables)
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]
The following tables set forth the computation of basic and diluted EPS from continuing operations for the periods indicated:
 
Year Ended December 31, 2016
 
Income
 
Shares
 
Per Share
Amount
 
(Thousands, except per share amounts)
Basic EPS Calculation
 
 
 
 
 
Net income available for common stock
$
140,095

 
52,453

 
$
2.67

Diluted EPS Calculation
 

 
 

 
 

Effect of dilutive securities
—

 
510

 
 

Net income available for common stock and common stock equivalents
$
140,095

 
52,963

 
$
2.65


 
Year Ended December 31, 2015
 
Income
 
Shares
 
Per Share
Amount
 
(Thousands, except per share amounts)
Basic EPS Calculation
 
 
 
 
 
Net income available for common stock
$
119,030

 
52,578

 
$
2.26

Diluted EPS Calculation
 
 
 

 
 

Effect of dilutive securities
—

 
676

 
 

Net income available for common stock and common stock equivalents
$
119,030

 
53,254

 
$
2.24


 
Year Ended December 31, 2014
 
Income
 
Shares
 
Per Share
Amount
 
(Thousands, except per share amounts)
Basic EPS Calculation
 
 
 
 
 
Net income available for common stock
$
109,790

 
52,364

 
$
2.10

Diluted EPS Calculation
 

 
 

 
 

Effect of dilutive securities
—

 
582

 
 

Net income available for common stock and common stock equivalents
$
109,790

 
52,946

 
$
2.07

REGULATORY ASSETS AND LIABILITIES (Tables)
SCHEDULE OF REGULATED ASSETS AND LIABILITIES
The table below presents a summary of regulatory assets, net of amortization, and liabilities for the periods indicated:
 
 
 
 
December 31, 2016
 
 
Remaining Recovery Period
 
Current
 
Noncurrent
 
Total
 
 
 
 
(Thousands of dollars)
Under-recovered purchased-gas costs
 
1 year
 
$
29,901

 
$
—

 
$
29,901

Pension and other postemployment benefit costs
 
See Note 11
 
31,498

 
427,448

 
458,946

Weather normalization
 
1 year
 
17,661

 
—

 
17,661

Reacquired debt costs
 
11 years
 
812

 
8,108

 
8,920

Other
 
1 to 22 years
 
3,274

 
4,966

 
8,240

Total regulatory assets, net of amortization
 
 
 
83,146

 
440,522

 
523,668

Over-recovered purchased-gas costs
 
1 year
 
(10,154
)
 
—

 
(10,154
)
Ad-valorem tax
 
1 year
 
(1,768
)
 
—

 
(1,768
)
Total regulatory liabilities
 
 
 
(11,922
)
 
—

 
(11,922
)
Net regulatory assets and liabilities
 
 
 
$
71,224

 
$
440,522

 
$
511,746


 
 
 
 
December 31, 2015
 
 
Remaining Recovery Period
 
Current
 
Noncurrent
 
Total
 
 
 
 
(Thousands of dollars)
Under-recovered purchased-gas costs
 
1 year
 
$
13,336

 
$
—

 
$
13,336

Pension and other postemployment benefit costs
 
See Note 11
 
15,670

 
425,175

 
440,845

Weather normalization
 
1 year
 
2,198

 
—

 
2,198

Reacquired debt costs
 
12 years
 
812

 
8,919

 
9,731

Other
 
1 to 23 years
 
909

 
1,769

 
2,678

Total regulatory assets, net of amortization
 
 
 
32,925

 
435,863

 
468,788

Accumulated removal costs (a)
 
up to 50 years
 
—

 
(9,032
)
 
(9,032
)
Over-recovered purchased-gas costs
 
1 year
 
(22,884
)
 
—

 
(22,884
)
Ad-valorem tax
 
1 year
 
(1,731
)
 
—

 
(1,731
)
Total regulatory liabilities
 
 
 
(24,615
)
 
(9,032
)
 
(33,647
)
Net regulatory assets and liabilities
 
 
 
$
8,310

 
$
426,831

 
$
435,141

(a) Included in other deferred credits in our Balance Sheets.
PROPERTY, PLANT AND EQUIPMENT (Tables) (Regulated [Member])
Property, Plant and Equipment by Property Type
The following table sets forth our property, plant and equipment by property type, for the periods indicated:
 
 
December 31,
 
December 31,
 
 
2016
 
2015
 
 
(Thousands of dollars)
Natural gas distribution pipelines and related equipment
 
$
4,321,429

 
$
4,114,090

Natural gas transmission pipelines and related equipment
 
481,953

 
462,654

General plant and other
 
530,459

 
498,906

Construction work in process
 
70,327

 
57,032

Property, plant and equipment
 
5,404,168

 
5,132,682

Accumulated depreciation and amortization
 
(1,672,548
)
 
(1,620,771
)
Net property, plant and equipment
 
$
3,731,620

 
$
3,511,911

SHARE-BASED PAYMENTS (Tables)
Schedule of Share-based Compensation, Restricted Stock Units Activity 
The following tables set forth activity and various statistics for restricted stock unit awards outstanding under the respective plans for the period indicated:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested December 31, 2015
 
231,258

 
$
32.59

Granted
 
42,935

 
$
58.30

Vested
 
(77,033
)
 
$
23.76

Forfeited
 
(2,260
)
 
$
38.04

Nonvested December 31, 2016
 
194,900

 
$
41.68

 
 
2016
 
2015
 
2014
Weighted-average grant date fair value (per share)
 
$
58.30

 
$
41.40

 
$
33.19

Fair value of shares granted (thousands of dollars)
 
$
2,503

 
$
3,141

 
$
3,149

Schedule of Nonvested Performance-based Units Activity 
The following tables set forth activity and various statistics related to our performance stock unit awards and the assumptions used by us in the valuations of the 2016, 2015 and 2014 grants at the grant date:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested December 31, 2015
 
439,250

 
$
27.35

Granted
 
74,395

 
$
64.06

Vested
 
(221,882
)
 
$
15.11

Forfeited
 
(2,952
)
 
$
41.44

Nonvested December 31, 2016
 
288,811

 
$
46.06

 
 
2016
 
2015
 
2014
 
Volatility (a)
 
18.20%
 
15.90%
 
18.40%
 
Dividend yield
 
2.40%
 
2.90%
 
3.37%
 
Risk-free interest rate
 
0.91%
 
1.10%
 
0.67%
 
(a) - Volatility based on historical volatility over three years using daily stock price observations of our peer utilities.
 
 
 
2016
 
2015
 
2014
Weighted-average grant date fair value (per share)
 
$
64.06

 
$
44.48

 
$
35.98

Fair value of shares granted (thousands of dollars)
 
$
4,766

 
$
4,486

 
$
4,462

EMPLOYEE BENEFIT PLANS (Tables)
Schedule of Assumptions Used [Table Text Block] 
Actuarial Assumptions - The following table sets forth the weighted-average assumptions used to determine benefit obligations for pension and postemployment benefits for the periods indicated:
 
 
December 31,
 
 
2016
 
2015
Discount rate - pension plans
 
4.30%
 
4.75%
Discount rate - other postemployment plans
 
4.20%
 
4.75%
Compensation increase rate
 
3.25% - 3.40%
 
3.35% - 3.40%

The following table sets forth the weighted-average assumptions used by us to determine the periodic benefit costs for the periods indicated:
 
 
Nine Months Ended September 30,
 
Three Months Ended December 31,
 
Years Ended December 31,
 
 
2016
 
2016
 
2015
 
 
2014
Discount rate - pension plans
 
4.75%
 
4.75%
 
4.25%/4.75%
(a)
 
5.25%
Discount rate - other postemployment plans
 
4.75%
 
3.75%
 
4.25%/4.75%
(a)
 
5.00%
Expected long-term return on plan assets - pension plans
 
7.75%
 
7.75%
 
7.75%
 
 
7.75%
Expected long-term return on plan assets - other postemployment plans
 
8.00%
 
7.75%
 
7.75%
 
 
7.75%
Compensation increase rate
 
3.35% - 3.40%
 
3.35% - 3.40%
 
3.30% - 3.50%
 
 
3.35% - 3.50%
Schedule of Defined Benefit Plans Disclosures [Table Text Block] 
The following table sets forth our defined benefit pension and other postemployment benefit plans, benefit obligations and fair value of plan assets for the periods indicated:

 
Pension Benefits
 
Other Postemployment Benefits
 
December 31,
 
December 31,
 
2016
 
2015
 
2016
 
2015
Changes in Benefit Obligation
(Thousands of dollars)
 
 
Benefit obligation, beginning of period
$
985,624

 
$
1,028,171

 
$
228,253

 
$
257,688

Service cost
12,055

 
13,660

 
2,675

 
3,257

Interest cost
45,550

 
43,542

 
10,235

 
10,628

Plan participants’ contributions
—

 
—

 
3,043

 
2,915

Actuarial loss (gain)
25,886

 
(47,607
)
 
14,309

 
(19,702
)
Benefits paid
(71,066
)
 
(52,142
)
 
(15,450
)
 
(14,632
)
Plan amendment
—

 
—

 
483

 
(11,901
)
Settlements
(31,518
)
 
—

 
—

 
—

   Benefit obligation, end of period
966,531

 
985,624

 
243,548

 
228,253

 
 
 
 
 
 
 
 
Change in Plan Assets
 
 
 
 
 
 
 
Fair value of plan assets, beginning of period
785,161

 
845,396

 
155,495

 
151,777

Actual return on plan assets
48,768

 
(9,026
)
 
9,733

 
1,335

Employer contributions
12,441

 
933

 
13,225

 
14,100

Plan participants’ contributions
—

 
—

 
3,043

 
2,915

Benefits paid
(71,066
)
 
(52,142
)
 
(15,450
)
 
(14,632
)
Settlements
(35,718
)
 
—

 
—

 
—

   Fair value of assets, end of period
739,586

 
785,161

 
166,046

 
155,495

   Balance at December 31
$
(226,945
)
 
$
(200,463
)
 
$
(77,502
)
 
$
(72,758
)
 
 
 
 
 
 
 
 
Current liabilities
$
(941
)
 
$
(912
)
 
$
—

 
$
—

Noncurrent liabilities
(226,004
)
 
(199,551
)
 
(77,502
)
 
(72,758
)
   Balance at December 31
$
(226,945
)
 
$
(200,463
)
 
$
(77,502
)
 
$
(72,758
)
Schedule of Net Benefit Costs [Table Text Block] 
Components of Net Periodic Benefit Cost - The following tables set forth the components of net periodic benefit cost for our defined benefit pension and other postemployment benefit plans for the period indicated:

 
Pension Benefits
 
Year Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
 
 
 
Service cost
$
12,055

 
$
13,660

 
$
11,620

Interest cost
45,550

 
43,542

 
43,791

Expected return on assets
(61,183
)
 
(61,769
)
 
(59,862
)
Amortization of unrecognized prior service cost
—

 
266

 
549

Amortization of net loss
35,543

 
42,226

 
30,200

Settlements
—

 
27

 
773

   Net periodic benefit cost
$
31,965

 
$
37,952

 
$
27,071


 
Other Postemployment Benefits
 
Year Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Components of net periodic benefit cost
 
 
 
 
 
Service cost
$
2,675

 
$
3,257

 
$
3,468

Interest cost
10,235

 
10,628

 
11,605

Expected return on assets
(12,370
)
 
(11,892
)
 
(11,393
)
Amortization of unrecognized prior service cost
(3,316
)
 
(2,228
)
 
(1,760
)
Amortization of net loss
5,369

 
5,268

 
3,969

   Net periodic benefit cost
$
2,593

 
$
5,033

 
$
5,889

Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block] 
Other Comprehensive Income (Loss) - The following table sets forth the amounts recognized in other comprehensive income (loss) related to our defined benefit pension benefits for the period indicated:

 
Pension Benefits
 
Year Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Net gain (loss) arising during the period
$
(1,262
)
 
$
339

 
$
(3,543
)
Amortization of loss
751

 
917

 
518

Deferred income taxes
197

 
(483
)
 
1,244

   Total recognized in other comprehensive income (loss)
$
(314
)
 
$
773

 
$
(1,781
)
Schedule of Net Periodic Benefit Cost Not yet Recognized [Table Text Block] 
The tables below set forth the amounts in accumulated other comprehensive income (loss) that had not yet been recognized as components of net periodic benefit expense for the periods indicated:

 
Pension Benefits
 
December 31,
 
2016
 
2015
 
(Thousands of dollars)
Prior service credit (cost)
$
—

 
$
—

Accumulated loss
(414,757
)
 
(407,798
)
Accumulated other comprehensive loss
  before regulatory assets
(414,757
)
 
(407,798
)
Regulatory asset for regulated entities
407,073

 
400,625

Accumulated other comprehensive loss
  after regulatory assets
(7,684
)
 
(7,173
)
Deferred income taxes
2,969

 
2,772

Accumulated other comprehensive loss,
  net of tax
$
(4,715
)
 
$
(4,401
)

 
Other Postemployment Benefits
 
December 31,
 
2016
 
2015
 
(Thousands of dollars)
Prior service credit (cost)
$
10,211

 
$
14,010

Accumulated loss
(62,084
)
 
(50,447
)
Accumulated other comprehensive loss
  before regulatory assets
(51,873
)
 
(36,437
)
Regulatory asset for regulated entities
51,873

 
36,437

Accumulated other comprehensive loss
  after regulatory assets
—

 
—

Deferred income taxes
—

 
—

Accumulated other comprehensive loss,
  net of tax
$
—

 
$
—

Schedule of Amounts in Accumulated Other Comprehensive Income (Loss) to be Recognized over Next Fiscal Year [Table Text Block] 
The following table sets forth the amounts recognized in either accumulated comprehensive income (loss) or regulatory assets expected to be recognized as components of net periodic benefit expense in the next fiscal year:

 
Pension Benefits
 
Other Postemployment Benefits
Amounts to be recognized in 2017
(Thousands of dollars)
Prior service credit (cost)
$
—

 
$
(4,597
)
Actuarial net loss
$
36,107

 
$
6,484

Schedule of Health Care Cost Trend Rates [Table Text Block] 
Health Care Cost Trend Rates - The following table sets forth the assumed health care cost-trend rates for the periods indicated:


2016
 
2015
Health care cost-trend rate assumed for next year
7.25%
 
4.00% - 7.50%
Rate to which the cost-trend rate is assumed to decline
  (the ultimate trend rate)
5.00%
 
4.00% - 5.00%
Year that the rate reaches the ultimate trend rate
2022
 
2022
Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates [Table Text Block] 
Assumed health care cost-trend rates have a significant effect on the amounts reported for our health care plans. A one percentage point change in assumed health care cost-trend rates would have the following effects:


One Percentage

One Percentage

Point Increase

Point Decrease

(Thousands of dollars)
Effect on total of service and interest cost
$
233


$
(232
)
Effect on other postemployment benefit obligation
$
3,937


$
(3,991
)
Schedule of Allocation of Plan Assets [Table Text Block] 
Plan Assets - Our investment strategy is to invest plan assets in accordance with sound investment practices that emphasize long-term fundamentals. The goal of this strategy is to maximize investment returns while managing risk in order to meet the plan’s current and projected financial obligations. To achieve this strategy, we have established a liability-driven investment strategy to change the allocations as the plan reaches certain funded status. The plan’s investments include a diverse blend of various domestic and international equities, investment-grade debt securities which mirror the cash flows of our liability, insurance contracts and alternative investments. The current target allocation for the assets of our defined benefit pension plan is as follows:
 
 
U.S. large-cap equities
37.4
%
Investment-grade bonds
30.0
%
Developed foreign large-cap equities
10.6
%
Alternative investments
7.7
%
Mid-cap equities
5.6
%
Emerging markets equities
5.0
%
Small-cap equities
3.7
%
  Total
100
%

As part of our risk management for the plans, minimums and maximums have been set for each of the asset classes listed above. All investment managers for the plan are subject to certain restrictions on the securities they purchase and, with the exception of indexing purposes, are prohibited from owning our stock.

The current target allocation for the assets of our other postemployment benefits plan is 30 percent fixed income securities and 70 percent equity securities.

The following tables set forth our pension benefits and other postemployment benefits plan assets by fair value category as of the measurement date:


Pension Benefits

December 31, 2016
Asset Category
Level 1
Level 2
Level 3
Total

(Thousands of dollars)
Investments:




Equity securities (a)
$
371,655

$
58,987

$
—

$
430,642

Government obligations
—

47,445

—

47,445

Corporate obligations (b)
—

129,036

—

129,036

Cash and money market funds (c)
13,786

16,114

—

29,900

Insurance contracts and group annuity contracts
—

—

45,140

45,140

Other investments (d)
—

71

57,352

57,423

  Total assets
$
385,441

$
251,653

$
102,492

$
739,586

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.
(d) - This category represents alternative investments such as hedge funds and other financial instruments.

 
Pension Benefits
 
December 31, 2015
Asset Category
Level 1
Level 2
Level 3
Total
 
(Thousands of dollars)
Investments:
 
 
 
 
Equity securities (a)
$
405,935

$
62,150

$
—

$
468,085

Government obligations
—

44,651

—

44,651

Corporate obligations (b)
—

139,396

—

139,396

Cash and money market funds (c)
5,429

10,279

—

15,708

Insurance contracts and group annuity contracts
—

—

56,465

56,465

Other investments (d)
2,884

—

57,972

60,856

  Total assets
$
414,248

$
256,476

$
114,437

$
785,161

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.
(d) - This category represents alternative investments such as hedge funds and other financial instruments.

 
Other Postemployment Benefits
 
December 31, 2016
Asset Category
Level 1
Level 2
Level 3
Total
 
(Thousands of dollars)
Investments:
 
 
 
 
Equity securities (a)
$
39,817

$
7,323

$
—

$
47,140

Government obligations
—

75

—

75

Corporate obligations (b)
—

19,948

—

19,948

Cash and money market funds (c)
74

16,989

—

17,063

Insurance contracts and group annuity contracts
—

81,820

—

81,820

  Total assets
$
39,891

$
126,155

$
—

$
166,046

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.


 
Other Postemployment Benefits
 
December 31, 2015
Asset Category
Level 1
Level 2
Level 3
Total
 
(Thousands of dollars)
Investments:
 
 
 
 
Equity securities (a)
$
54,560

$
7,498

$
—

$
62,058

Government obligations
—

64

—

64

Corporate obligations (b)
—

200

—

200

Cash and money market funds (c)
233

13,322

—

13,555

Insurance contracts and group annuity contracts
—

79,531

—

79,531

Other investments (d)
4

—

83

87

  Total assets
$
54,797

$
100,615

$
83

$
155,495

(a) - This category represents securities of the various market sectors from diverse industries.
(b) - This category represents bonds from diverse industries.
(c) - This category is primarily money market funds.
(d) - This category represents alternative investments such as hedge funds.

Schedule of Effect of Significant Unobservable Inputs, Changes in Plan Assets [Table Text Block] 
The following table sets forth the reconciliation of Level 3 fair value measurements of our pension plans for the periods indicated:

 
Pension Benefits
 
Insurance
Contracts
 
Other
Investments
 
Total
 
(Thousands of dollars)
January 1, 2015
$
59,877

 
$
57,914

 
$
117,791

Net realized and unrealized gains (losses)
2,188

 
58

 
2,246

Settlements
(5,600
)
 
—

 
(5,600
)
December 31, 2015
$
56,465

 
$
57,972

 
$
114,437

Net realized and unrealized gains (losses)
4,518

 
(620
)
 
3,898

Sales and settlements
(15,843
)
 
—

 
(15,843
)
December 31, 2016
$
45,140

 
$
57,352

 
$
102,492

Schedule of Expected Benefit Payments [Table Text Block] 
The following table sets forth the pension benefits and other postemployment benefits payments expected to be paid in 2017-2026:

 
Pension
Benefits
 
Other Postemployment
Benefits
Benefits to be paid in:
(Thousands of dollars)
2017
$
51,539

 
$
16,165

2018
$
52,660

 
$
16,815

2019
$
53,450

 
$
17,073

2020
$
54,812

 
$
17,379

2021
$
56,033

 
$
17,401

2022 through 2026
$
294,519

 
$
86,559

INCOME TAXES (Tables)
Reconciliation of Income Tax Provision 
The following table sets forth our provision for income taxes for the periods indicated:

 
Years Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Current income tax provision
 
 
 
 
 
Federal
$
(2,016
)
 
$
7,135

 
$
17,006

State
471

 
2,055

 
1,397

Total current income tax provision
(1,545
)
 
9,190

 
18,403

Deferred income tax provision
 
 
 
 
 
Federal
76,247

 
56,440

 
42,024

State
10,541

 
7,349

 
7,911

Total deferred income tax provision
86,788

 
63,789

 
49,935

Total provision for income taxes
$
85,243

 
$
72,979

 
$
68,338

Schedule of Effective Income Tax Rate Reconciliation 
The following table is a reconciliation of our income tax provision for the periods indicated:
 
Years Ended December 31,
 
2016
 
2015
 
2014
 
(Thousands of dollars)
Income before income taxes
$
225,338

 
$
192,009

 
$
178,128

Federal statutory income tax rate
35
%
 
35
%
 
35
%
Provision for federal income taxes
78,868

 
67,203

 
62,345

State income taxes, net of federal tax benefit
7,158

 
6,114

 
6,051

Other, net
(783
)
 
(338
)
 
(58
)
Total provision for income taxes
$
85,243

 
$
72,979

 
$
68,338

Schedule of Deferred Tax Assets and Liabilities 
The following table sets forth the tax effects of temporary differences that gave rise to significant portions of the deferred tax assets and liabilities for the periods indicated:
 
December 31,
 
2016
 
2015
 
(Thousands of dollars)
Deferred tax assets
 
 
 
Employee benefits and other accrued liabilities
$
123,333

 
$
110,148

Net operating loss
23,094

 
—

Other
5,716

 
7,848

Total deferred tax assets
152,143

 
117,996

Deferred tax liabilities
 
 
 
Excess of tax over book depreciation
990,682

 
897,667

Purchased-gas cost adjustment
13,822

 
3,999

Other regulatory assets and liabilities, net
186,207

 
168,115

Total deferred tax liabilities
1,190,711

 
1,069,781

Net deferred tax liabilities
$
1,038,568

 
$
951,785

COMMITMENTS AND CONTINGENCIES (Tables)
Future Minimum Rental Payments for Operating Leases
The following table sets forth our operating lease payments for the periods indicated:
Operating Leases
(Millions of dollars)
2017
 
$
5.6

2018
 
5.2

2019
 
4.4

2020
 
3.6

2021
 
3.2

Thereafter
 
4.4

Total
 
$
26.4

QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables)
Quarterly Financial Data
 
 
First
Quarter
 
Second
Quarter
 
Third
Quarter
 
Fourth
Quarter
Year Ended December 31, 2016
 
 
 
 
 
 
(Thousands of dollars)
Revenues
 
$
508,364

 
$
245,923

 
$
232,191

 
$
440,754

Operating income
 
$
116,073

 
$
43,621

 
$
30,892

 
$
78,534

Net income
 
$
64,743

 
$
20,300

 
$
12,737

 
$
42,315

Earnings per share
 
 
 
 
 
 
 
 
   Basic
 
$
1.23

 
$
0.39

 
$
0.24

 
$
0.81

   Diluted
 
$
1.22

 
$
0.38

 
$
0.24

 
$
0.80

 
 
First
Quarter
 
Second
Quarter
 
Third
Quarter
 
Fourth
Quarter
Year Ended December 31, 2015
 
 
 
 
 
 
(Thousands of dollars)
Revenues
 
$
676,531

 
$
256,786

 
$
225,226

 
$
389,149

Operating income
 
$
109,005

 
$
31,270

 
$
24,951

 
$
73,903

Net income
 
$
60,381

 
$
12,076

 
$
7,371

 
$
39,202

Earnings per share
 
 
 
 
 
 
 
 
   Basic
 
$
1.15

 
$
0.23

 
$
0.14

 
$
0.75

   Diluted
 
$
1.13

 
$
0.23

 
$
0.14

 
$
0.74

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Significant Accounting Policies [Line Items]
 
 
 
Operating Loss Carryforwards, Tax Benefit
$ 11,000,000 
 
 
Unbilled Receivables, Current
143,200,000 
109,600,000 
 
Number of natural gas distribution services customers
2,000,000 
 
 
Allowance for Doubtful Accounts Receivable, Current
4,200,000 
3,500,000 
 
Materials, Supplies, and Other
34,084,000 
33,325,000 
 
Goodwill
157,953,000 
157,953,000 
 
Goodwill, Impairment Loss
0 
0 
0 
Asset Impairment Charges
0 
0 
0 
Deferred Tax Assets, Valuation Allowance
0 
0 
 
Liability for Uncertain Tax Positions, Current
$ 0 
$ 0 
 
Segment Reporting, Disclosure of Major Customers
0 
0 
0 
CREDIT FACILITY AND SHORT-TERM NOTES PAYABLE (Details) (USD $)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Line of Credit Facility [Line Items]
 
 
Ratio of Indebtedness to Net Capital
0.41 
 
Commercial paper maximum borrowing capacity
$ 700,000,000 
 
Commercial Paper
145,000,000 
12,500,000 
Letters of Credit Outstanding, Amount
1,500,000 
 
Short-term Debt
0 
 
Line of Credit Facility, Remaining Borrowing Capacity
553,500,000 
 
Short-term Debt, Weighted Average Interest Rate
0.95% 
0.70% 
Line of Credit [Member]
 
 
Line of Credit Facility [Line Items]
 
 
Debt Instrument, Covenant Description
The ONE Gas Credit Agreement contains certain financial, operational and legal covenants. Among other things, these covenants include maintaining ONE Gas’ total debt-to-capital ratio of no more than 70 percent at the end of any calendar quarter. The ONE Gas Credit Agreement also contains customary affirmative and negative covenants, including covenants relating to liens, indebtedness of subsidiaries, investments, changes in the nature of business, fundamental changes, transactions with affiliates, burdensome agreements, and use of proceeds. In the event of a breach of certain covenants by ONE Gas, amounts outstanding under the ONE Gas Credit Agreement may become due and payable immediately. 
 
Line of credit facility sublimit
50,000,000 
 
Line Of Credit Facility Option To Increase Borrowing Capacity
1,200,000,000 
 
Line of Credit Facility, Maximum Borrowing Capacity
$ 700,000,000 
 
Line of Credit Facility, Interest Rate Description
Borrowings, if any, will accrue interest at LIBOR plus 79.5 basis points. 
 
Line of Credit Facility, Commitment Fee Description
The annual facility fee is 8 basis points. 
 
LONG-TERM DEBT (Details) (USD $)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]
 
 
 
Cash Payment to ONEOK Upon Separation
$ 0 
$ 0 
$ 1,130,000,000 
Note Payable Due 2019 [Member]
 
 
 
Debt Instrument [Line Items]
 
 
 
Debt Instrument, Covenant Description
The indenture governing our Senior Notes includes an event of default upon the acceleration of other indebtedness of $100 million or more. Such events of default would entitle the trustee or the holders of 25 percent in the aggregate principal amount of the outstanding Senior Notes to declare those senior notes immediately due and payable in full. 
 
 
Long-term Debt, Gross
300,000,000 
 
 
Debt Instrument, Interest Rate, Stated Percentage
2.07% 
 
 
Debt Instrument, Call Feature
We may redeem our Senior Notes at par, plus accrued and unpaid interest to the redemption date, starting one month, three months, and six months, respectively, before their maturity dates. Prior to these dates, we may redeem these Senior Notes, in whole or in part, at a redemption price equal to the principal amount, plus accrued and unpaid interest and a make-whole premium. The redemption price will never be less than 100 percent of the principal amount of the respective note plus accrued and unpaid interest to the redemption date. Our Senior Notes are senior unsecured obligations, ranking equally in right of payment with all of our existing and future unsecured senior indebtedness. 
 
 
Note Payable Due 2024 [Member]
 
 
 
Debt Instrument [Line Items]
 
 
 
Debt Instrument, Covenant Description
The indenture governing our Senior Notes includes an event of default upon the acceleration of other indebtedness of $100 million or more. Such events of default would entitle the trustee or the holders of 25 percent in the aggregate principal amount of the outstanding Senior Notes to declare those senior notes immediately due and payable in full. 
 
 
Long-term Debt, Gross
300,000,000 
 
 
Debt Instrument, Interest Rate, Stated Percentage
3.61% 
 
 
Debt Instrument, Call Feature
We may redeem our Senior Notes at par, plus accrued and unpaid interest to the redemption date, starting one month, three months, and six months, respectively, before their maturity dates. Prior to these dates, we may redeem these Senior Notes, in whole or in part, at a redemption price equal to the principal amount, plus accrued and unpaid interest and a make-whole premium. The redemption price will never be less than 100 percent of the principal amount of the respective note plus accrued and unpaid interest to the redemption date. Our Senior Notes are senior unsecured obligations, ranking equally in right of payment with all of our existing and future unsecured senior indebtedness. 
 
 
Notes Payable Due 2044 [Member]
 
 
 
Debt Instrument [Line Items]
 
 
 
Debt Instrument, Covenant Description
The indenture governing our Senior Notes includes an event of default upon the acceleration of other indebtedness of $100 million or more. Such events of default would entitle the trustee or the holders of 25 percent in the aggregate principal amount of the outstanding Senior Notes to declare those senior notes immediately due and payable in full.  
 
 
Long-term Debt, Gross
$ 600,000,000 
 
 
Debt Instrument, Interest Rate, Stated Percentage
4.658% 
 
 
Debt Instrument, Call Feature
We may redeem our Senior Notes at par, plus accrued and unpaid interest to the redemption date, starting one month, three months, and six months, respectively, before their maturity dates. Prior to these dates, we may redeem these Senior Notes, in whole or in part, at a redemption price equal to the principal amount, plus accrued and unpaid interest and a make-whole premium. The redemption price will never be less than 100 percent of the principal amount of the respective note plus accrued and unpaid interest to the redemption date. Our Senior Notes are senior unsecured obligations, ranking equally in right of payment with all of our existing and future unsecured senior indebtedness. 
 
 
EQUITY (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
12 Months Ended 3 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Mar. 31, 2017
Dividend Declared [Member]
Dividends declared per share of stock
$ 1.40 
$ 1.20 
$ 0.84 
 
Preferred Stock, Shares Authorized
50 
 
 
 
Preferred Stock, Par or Stated Value Per Share
$ 0.01 
 
 
 
Common stock authorized and available for issuance
197.7 
 
 
 
Stock Repurchase Program, Authorized Amount
$ 20 
 
 
 
Common Stock, Dividends, Per Share, Declared
 
 
 
$ 0.42 
Common Stock, Dividends, Declared, Annualized Basis
$ 1.40 
$ 1.20 
$ 0.84 
$ 1.68 
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
 
 
 
Pension and other postretirement benefit plans, Tax
$ 486 
$ (130)
 
Accumulated Other Comprehensive Income (Loss), beginning balance
(4,401)
(5,174)
 
Pension and other postretirement benefit plans obligations [Abstract]
 
 
 
Other comprehensive income (loss), before reclassification, net of tax
(776)
209 
 
Amounts reclassified from accumulated other comprehensive income (loss), net of tax
462 
564 
 
Other comprehensive loss
(314)
773 
(1,781)
Accumulated Other Comprehensive Income (Loss), ending balance
(4,715)
(4,401)
(5,174)
Pension and other postretirement benefit plans obligations [Abstract]
 
 
 
Amortization of net loss
40,912 
47,494 
34,169 
Amortization of unrecognized prior service cost
(3,316)
(1,962)
(1,211)
Other comprehensive income (loss) reclassification adjustment, before tax and regulatory adjustments
37,596 
45,532 
32,958 
Other comprehensive income (loss) reclassification - regulatory adjustments
(36,845)
(44,615)
(32,445)
Other comprehensive income (loss) reclassification adjustment, before tax
751 
917 
513 
Other comprehensive income (loss) reclassification adjustment, Tax
(289)
(353)
(198)
Other comprehensive income (loss) reclassification adjustment, net of tax
$ 462 
$ 564 
$ 315 
EARNINGS PER SHARE (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
EARNINGS PER SHARE OF COMMON STOCK, BASIC AND DILUTED [Line Items]
 
 
 
 
 
 
 
 
 
 
 
Net income
$ 42,315 
$ 12,737 
$ 20,300 
$ 64,743 
$ 39,202 
$ 7,371 
$ 12,076 
$ 60,381 
$ 140,095 
$ 119,030 
$ 109,790 
Weighted Average Number of Shares Outstanding, Basic
 
 
 
 
 
 
 
 
52,453,000 
52,578,000 
52,364,000 
Earnings Per Share, Basic
$ 0.81 
$ 0.24 
$ 0.39 
$ 1.23 
$ 0.75 
$ 0.14 
$ 0.23 
$ 1.15 
$ 2.67 
$ 2.26 
$ 2.10 
Dilutive Securities, Effect on Basic Earnings Per Share
 
 
 
 
 
 
 
 
0 
0 
0 
Weighted Average Number Diluted Shares Outstanding Adjustment
 
 
 
 
 
 
 
 
510,000 
676,000 
582,000 
Net Income (Loss) Available to Common Stockholders, Diluted
 
 
 
 
 
 
 
 
$ 140,095 
$ 119,030 
$ 109,790 
Weighted Average Number of Shares Outstanding, Diluted
 
 
 
 
 
 
 
 
52,963,000 
53,254,000 
52,946,000 
Earnings Per Share, Diluted
$ 0.80 
$ 0.24 
$ 0.38 
$ 1.22 
$ 0.74 
$ 0.14 
$ 0.23 
$ 1.13 
$ 2.65 
$ 2.24 
$ 2.07 
Common Stock, Shares, Issued
52,598,005 
 
 
 
52,598,005 
 
 
 
52,598,005 
52,598,005 
 
DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details) (USD $)
12 Months Ended
Dec. 31, 2016
MMcf
Dec. 31, 2015
MMcf
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
 
 
Derivative, Nonmonetary Notional Amount
14,300 
17,000 
Premiums recorded in other current assets on natural gas contracts held
$ 5,400,000 
$ 5,800,000 
Fair Value Assets, Transfers between Levels
0 
0 
Long-term Debt
1,200,000,000 
1,200,000,000 
Fair Value, Inputs, Level 1 [Member]
 
 
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
 
 
Fair value of natural gas call options held
6,500,000 
400,000 
Fair Value, Inputs, Level 2 [Member]
 
 
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
 
 
Long-term Debt, Fair Value
$ 1,200,000,000 
$ 1,200,000,000 
REGULATORY ASSETS AND LIABILITIES (Details) (USD $)
12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2015
Accumulated removal costs [Member]
Dec. 31, 2016
Weather normalization [Member]
Dec. 31, 2015
Weather normalization [Member]
Dec. 31, 2016
Over-recovered purchased-gas costs [Member]
Dec. 31, 2015
Over-recovered purchased-gas costs [Member]
Dec. 31, 2016
Ad valorem tax [Member]
Dec. 31, 2015
Ad valorem tax [Member]
Dec. 31, 2016
Total regulated liabilities [Member]
Dec. 31, 2015
Total regulated liabilities [Member]
Dec. 31, 2016
Under-recovered purchased-gas costs [Member]
Dec. 31, 2015
Under-recovered purchased-gas costs [Member]
Dec. 31, 2016
Pension and postretirement benefit costs [Member]
Dec. 31, 2015
Pension and postretirement benefit costs [Member]
Dec. 31, 2016
Reacquired debt costs [Member]
Dec. 31, 2015
Reacquired debt costs [Member]
Dec. 31, 2016
Other regulatory assets [Member]
Dec. 31, 2015
Other regulatory assets [Member]
Dec. 31, 2016
Total regulatory assets, net of amortization [Member]
Dec. 31, 2015
Total regulatory assets, net of amortization [Member]
Dec. 31, 2016
Information Technology Costs [Member]
Jan. 31, 2016
Information Technology Costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Regulatory Asset, Amortization Period
 
 
 
up to 50 years 
1 year 
1 year 
1 year 
1 year 
1 year 
1 year 
 
 
1 year 
1 year 
See Note 11 
See Note 11 
11 years 
12 years 
1 to 22 years 
1 to 23 years 
 
 
4 
 
Regulatory Assets, Current
$ 83,146,000 
$ 32,925,000 
 
 
$ 17,661,000 
$ 2,198,000 
 
 
 
 
 
 
$ 29,901,000 
$ 13,336,000 
$ 31,498,000 
$ 15,670,000 
$ 812,000 
$ 812,000 
$ 3,274,000 
$ 909,000 
$ 83,146,000 
$ 32,925,000 
 
 
Regulatory Assets, Noncurrent
440,522,000 
435,863,000 
 
 
0 
0 
 
 
 
 
 
 
0 
0 
427,448,000 
425,175,000 
8,108,000 
8,919,000 
4,966,000 
1,769,000 
440,522,000 
435,863,000 
 
2,400,000 
Regulatory Assets
 
 
 
 
17,661,000 
2,198,000 
 
 
 
 
 
 
29,901,000 
13,336,000 
458,946,000 
440,845,000 
8,920,000 
9,731,000 
8,240,000 
2,678,000 
523,668,000 
468,788,000 
 
 
Regulatory Liability, Current
(11,922,000)
(24,615,000)
 
0 
 
 
(10,154,000)
(22,884,000)
(1,768,000)
(1,731,000)
(11,922,000)
(24,615,000)
 
 
 
 
 
 
 
 
 
 
 
 
Regulatory Liability, Noncurrent
 
 
 
(9,032,000)
 
 
0 
0 
0 
0 
0 
(9,032,000)
 
 
 
 
 
 
 
 
 
 
 
 
Regulatory Liabilities
 
 
 
(9,032,000)
 
 
(10,154,000)
(22,884,000)
(1,768,000)
(1,731,000)
(11,922,000)
(33,647,000)
 
 
 
 
 
 
 
 
 
 
 
 
Net regulatory assets and liabilities, current
71,224,000 
8,310,000 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Net regulatory assets and liabilities, noncurrent
440,522,000 
426,831,000 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Net Regulatory Assets
511,746,000 
435,141,000 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Amortization of Rate Deferral
$ 3,800,000 
$ 1,600,000 
$ 6,400,000 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PROPERTY, PLANT AND EQUIPMENT (Details) (USD $)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Property, Plant and Equipment [Line Items]
 
 
 
Property, plant and equipment
$ 5,404,168,000 
$ 5,132,682,000 
 
Accumulated depreciation and amortization
(1,672,548,000)
(1,620,771,000)
 
Net property, plant and equipment
3,731,620,000 
3,511,911,000 
 
Regulated [Member]
 
 
 
Property, Plant and Equipment [Line Items]
 
 
 
Property, plant and equipment
5,404,168,000 
5,132,682,000 
 
Accumulated depreciation and amortization
(1,672,548,000)
(1,620,771,000)
 
Net property, plant and equipment
3,731,620,000 
3,511,911,000 
 
Average depreciation rates, minimum
2.00% 
2.00% 
2.00% 
Average depreciation rates, maximum
3.00% 
3.00% 
3.00% 
Interest costs capitalized
3,600,000 
2,600,000 
2,500,000 
Construction work in process expenditures incurred but not yet paid
11,900,000 
15,000,000 
7,000,000 
Natural gas distribution pipelines and related equipment |
Regulated [Member]
 
 
 
Property, Plant and Equipment [Line Items]
 
 
 
Property, plant and equipment
4,321,429,000 
4,114,090,000 
 
Natural gas transmission pipelines and related equipment |
Regulated [Member]
 
 
 
Property, Plant and Equipment [Line Items]
 
 
 
Property, plant and equipment
481,953,000 
462,654,000 
 
General plant and other |
Regulated [Member]
 
 
 
Property, Plant and Equipment [Line Items]
 
 
 
Property, plant and equipment
530,459,000 
498,906,000 
 
Construction work in process |
Regulated [Member]
 
 
 
Property, Plant and Equipment [Line Items]
 
 
 
Property, plant and equipment
$ 70,327,000 
$ 57,032,000 
 
SHARE-BASED PAYMENTS (Details) (USD $)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Payments [Line Items]
 
 
 
Common Stock, Capital Shares Reserved for Future Issuance
2,800,000 
 
 
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant
1,100,000 
 
 
Share-based compensation expense, net of tax
$ 7,000,000 
$ 5,700,000 
$ 7,000,000 
Share-based compensation, tax benefit
4,300,000 
3,500,000 
4,400,000 
Shares remaining
15603 
 
 
Employee Stock Award Program Compensation Expense
11,219,000 
9,187,000 
7,613,000 
Employee Stock Purchase Plan [Member]
 
 
 
Share-based Payments [Line Items]
 
 
 
Description
Subject to certain exclusions, all employees who work at least 20 hours per week are eligible to participate in the ESPP.  Employees can choose to have up to 10 percent of their annual base pay withheld to purchase our common stock, subject to terms and limitations of the plan. The purchase price of the stock is 85 percent of the lower of the average market price of our common stock on the grant date or exercise date. 
 
 
Employee Stock Award Program [Member]
 
 
 
Share-based Payments [Line Items]
 
 
 
Description
Under the program, each time the per-share closing price of our common stock on the NYSE closed for the first time at or above each $1.00 increment above its previous historical high closing price, we issued, for no monetary consideration, one share of our common stock to all eligible employees. 
 
 
Restricted Stock Units (RSUs) [Member]
 
 
 
Share-based Payments [Line Items]
 
 
 
Award vesting period
3 years 
 
 
Forfeiture rate maximum (in hundredths)
3.00% 
 
 
Total compensation cost not yet recognized
2,800,000 
 
 
Total compensation cost not yet recognized, period for recognition
1 year 8 months 18 days 
 
 
Fair value of vested shares
4,500,000 
6,500,000 
 
Nonvested beginning balance (in units)
231,258 
 
 
Nonvested beginning balance (in dollars per unit)
$ 32.59 
 
 
Granted (in units)
42,935 
 
 
Weighted -average grant date fair value (per unit)
$ 58.30 
$ 41.40 
$ 33.19 
Vested (in units)
(77,033)
 
 
Vested (in dollars per unit)
$ 23.76 
 
 
Forfeited (in units)
(2,260)
 
 
Forfeited (in dollars per unit)
$ 38.04 
 
 
Nonvested ending balance (in units)
194,900 
231,258 
 
Nonvested ending balance (in dollars per unit)
$ 41.68 
$ 32.59 
 
Fair value of shares granted (thousands of dollars)
2,503,000 
3,141,000 
3,149,000 
Performance Unit Awards [Member]
 
 
 
Share-based Payments [Line Items]
 
 
 
Award vesting period
3 years 
 
 
Forfeiture rate maximum (in hundredths)
3.00% 
 
 
Total compensation cost not yet recognized
4,800,000 
 
 
Total compensation cost not yet recognized, period for recognition
1 year 8 months 12 days 
 
 
Fair value of vested shares
19,500,000 
23,500,000 
 
Nonvested beginning balance (in units)
439,250 
 
 
Nonvested beginning balance (in dollars per unit)
$ 27.35 
 
 
Granted (in units)
74,395 
 
 
Weighted -average grant date fair value (per unit)
$ 64.06 
$ 44.48 
$ 35.98 
Vested (in units)
(221,882)
 
 
Vested (in dollars per unit)
$ 15.11 
 
 
Forfeited (in units)
(2,952)
 
 
Forfeited (in dollars per unit)
$ 41.44 
 
 
Nonvested ending balance (in units)
288,811 
439,250 
 
Nonvested ending balance (in dollars per unit)
$ 46.06 
$ 27.35 
 
Fair value of shares granted (thousands of dollars)
4,766,000 
4,486,000 
4,462,000 
Expected volatility rate
18.20% 
15.90% 
18.40% 
Expected dividend yield
2.40% 
2.90% 
3.37% 
Risk-free interest rate
0.91% 
1.10% 
0.67% 
Description
Upon vesting, a holder of performance stock units is entitled to receive a number of shares of common stock equal to a percentage (0 percent to 200 percent) of the performance stock units granted, based on our total shareholder return over the vesting period, compared with the total shareholder return of a peer group of other utilities over the same period. 
 
 
Employee Stock Purchase Plan [Member]
 
 
 
Share-based Payments [Line Items]
 
 
 
Common Stock, Capital Shares Reserved for Future Issuance
700,000 
 
 
Maximum allowable percentage of annual base pay withheld to purchase common stock
10.00% 
 
 
Purchase price percentage of the lower of its grant date or exercise date market price (in hundredths)
85.00% 
 
 
Percent of employees who participated in the Employee Stock Purchase Plan
41.00% 
40.00% 
36.00% 
Shares sold under employee stock purchase plan
83,431 
51,092 
51,418 
Share price of shares sold under Employee Stock Purchase Plan in dollars per share
$ 54.51 
$ 36.15 
$ 32.29 
Employee Stock Purchase Plan Compensation Expense
1,400,000 
1,300,000 
400,000 
Employee Stock Award Program [Member]
 
 
 
Share-based Payments [Line Items]
 
 
 
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized
125,000 
 
 
Shares issued to employees under the Employee Stock Award Program
50,573 
23,506 
35,324 
Employee Stock Award Program Compensation Expense
$ 3,000,000 
$ 1,100,000 
$ 2,500,000 
EMPLOYEE BENEFIT PLANS (Details) (USD $)
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Minimum
3.25% 
 
3.25% 
3.35% 
 
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Maximum
3.40% 
 
3.40% 
3.40% 
 
Compensation increase rate - minimum
3.35% 
3.35% 
 
3.30% 
3.35% 
Compensation increase rate- maximum
3.40% 
3.40% 
 
3.50% 
3.50% 
Description of basis used to determine overall expected long-term rate of return on plan assets
 
 
We determine our overall expected long-term rate of return on plan assets, based on our review of historical returns and economic growth models. As of December 31, 2014, we updated our assumed mortality rates to incorporate the new set of mortality tables issued by the Society of Actuaries in October 2014. We determine our discount rates annually.  We estimate our discount rate based upon a comparison of the expected cash flows associated with our future payments under our defined benefit pension and other postemployment obligations to a hypothetical bond portfolio created using high-quality bonds that closely match expected cash flows.  Bond portfolios are developed by selecting a bond for each of the next 60 years based on the maturity dates of the bonds.  Bonds selected to be included in the portfolios are only those rated by Moody’s as AA- or better and exclude callable bonds, bonds with less than a minimum issue size, yield outliers and other filtering criteria to remove unsuitable bonds. 
 
 
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
 
 
 
 
 
Benefits paid
$ (18,100,000)
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Benefits paid
(18,100,000)
 
 
 
 
Noncurrent liabilities
(303,507,000)
 
(303,507,000)
(272,309,000)
 
Benefits paid
(18,100,000)
 
 
 
 
Assets expected to be withdrawn or returned
 
 
0 
 
 
Amortization of unrecognized prior service cost
 
 
(3,316,000)
(1,962,000)
(1,211,000)
Amortization of loss
 
 
40,912,000 
47,494,000 
34,169,000 
Total recognized in other comprehensive income (loss)
 
 
(314,000)
773,000 
(1,781,000)
Amount recognized in other comprehensive income
 
 
0 
 
 
Health care cost-trend rate assumed for next year
 
 
7.25% 
 
 
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)
 
 
5.00% 
 
 
Year that the rate reaches the ultimate trend rate
2022 
 
2022 
2022 
 
Target asset allocation
 
 
100.00% 
 
 
US Large-Cap Equity [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
37.40% 
 
 
Investment-grade bonds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
30.00% 
 
 
Developed foreign large-cap equities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
10.60% 
 
 
Alternative investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
7.70% 
 
 
Mid-cap equities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
5.60% 
 
 
Emerging market equities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
5.00% 
 
 
Small-cap equities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Target asset allocation
 
 
3.70% 
 
 
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Defined Benefit Plan, Target Allocation Percentage
 
 
.7 
 
 
Fixed Income Funds [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Defined Benefit Plan, Target Allocation Percentage
 
 
.3 
 
 
Pension Plan [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Net periodic benefit cost
 
 
31,965,000 
37,952,000 
27,071,000 
Plan amendment
 
 
0 
0 
 
Discount rate
4.30% 
 
4.30% 
4.75% 
 
Weighted average discount rate
4.75% 
4.75% 
 
 
5.25% 
Expected long-term return on plan assets
7.75% 
7.75% 
 
7.75% 
7.75% 
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
 
 
 
 
 
Benefit obligation, beginning of period
 
985,624,000 
985,624,000 
1,028,171,000 
 
Service cost
 
 
12,055,000 
13,660,000 
11,620,000 
Interest cost
 
 
45,550,000 
43,542,000 
43,791,000 
Plan participants' contributions
 
 
0 
0 
 
Actuarial loss (gain)
 
 
25,886,000 
(47,607,000)
 
Plan amendment
 
 
0 
0 
 
Benefits paid
 
 
(71,066,000)
(52,142,000)
 
Settlements
 
 
(31,518,000)
0 
 
Benefit obligation, end of period
966,531,000 
 
966,531,000 
985,624,000 
1,028,171,000 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, beginning of period
 
785,161,000 
785,161,000 
845,396,000 
 
Actual return on plan assets
 
 
48,768,000 
(9,026,000)
 
Employer contributions
 
 
12,441,000 
933,000 
 
Plan participants' contributions
 
 
0 
0 
 
Benefits paid
 
 
(71,066,000)
(52,142,000)
 
Settlements
 
 
(35,718,000)
0 
 
Fair value of plan assets, end of period
739,586,000 
 
739,586,000 
785,161,000 
845,396,000 
Balance at December 31
(226,945,000)
 
(226,945,000)
(200,463,000)
 
Current liabilities
(941,000)
 
(941,000)
(912,000)
 
Noncurrent liabilities
(226,004,000)
 
(226,004,000)
(199,551,000)
 
Balance at December 31
(226,945,000)
 
(226,945,000)
(200,463,000)
 
Benefits paid
 
 
(71,066,000)
(52,142,000)
 
Accumulated benefit obligation
912,400,000 
 
912,400,000 
934,300,000 
 
Service cost
 
 
12,055,000 
13,660,000 
11,620,000 
Interest cost
 
 
45,550,000 
43,542,000 
43,791,000 
Expected return on assets
 
 
(61,183,000)
(61,769,000)
(59,862,000)
Amortization of unrecognized prior service cost
 
 
0 
266,000 
549,000 
Amortization of loss
 
 
35,543,000 
42,226,000 
30,200,000 
Settlements
 
 
0 
27,000 
773,000 
Net periodic benefit cost
 
 
31,965,000 
37,952,000 
27,071,000 
Net loss arising during the period
 
 
(1,262,000)
339,000 
(3,543,000)
Amortization of loss
 
 
751,000 
917,000 
518,000 
Deferred income taxes
 
 
197,000 
(483,000)
1,244,000 
Total recognized in other comprehensive income (loss)
 
 
314,000 
(773,000)
1,781,000 
Prior service credit (cost)
0 
 
0 
0 
 
Accumulated loss
(414,757,000)
 
(414,757,000)
(407,798,000)
 
Accumulated other comprehensive loss before regulatory assets
(414,757,000)
 
(414,757,000)
(407,798,000)
 
Regulatory asset for regulated entities
407,073,000 
 
407,073,000 
400,625,000 
 
Accumulated other comprehensive loss after regulatory assets
(7,684,000)
 
(7,684,000)
(7,173,000)
 
Deferred income taxes
2,969,000 
 
2,969,000 
2,772,000 
 
Accumulated other comprehensive loss, net of tax
(4,715,000)
 
(4,715,000)
(4,401,000)
 
Prior service credit (cost) expected to be recognized in following fiscal year
0 
 
0 
 
 
Actuarial net loss, to be recognized in 2017
36,107,000 
 
36,107,000 
 
 
Effect of a one percentage point change on total of service and interest cost - increase
 
 
233,000 
 
 
Effect of a one percentage point change on other postemployment benefit obligation - increase
 
 
3,937,000 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Level 3 fair value measurement, January 1
 
114,437,000 
114,437,000 
117,791,000 
 
Net realized and unrealized gains (losses)
 
 
3,898,000 
2,246,000 
 
Settlements
 
 
(15,843,000)
(5,600,000)
 
Level 3 fair value measurement, December 31
102,492,000 
 
102,492,000 
114,437,000 
117,791,000 
Employer contributions
 
 
12,441,000 
933,000 
 
Expected employer contributions in 2017
 
 
1,000,000 
 
 
2017
51,539,000 
 
51,539,000 
 
 
2018
52,660,000 
 
52,660,000 
 
 
2019
53,450,000 
 
53,450,000 
 
 
2020
54,812,000 
 
54,812,000 
 
 
2021
56,033,000 
 
56,033,000 
 
 
2022 through 2027
294,519,000 
 
294,519,000 
 
 
Pension Plan [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
430,642,000 
 
430,642,000 
468,085,000 
 
Pension Plan [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
47,445,000 
 
47,445,000 
44,651,000 
 
Pension Plan [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
129,036,000 
 
129,036,000 
139,396,000 
 
Pension Plan [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
29,900,000 
 
29,900,000 
15,708,000 
 
Pension Plan [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, beginning of period
 
56,465,000 
56,465,000 
 
 
Fair value of plan assets, end of period
45,140,000 
 
45,140,000 
56,465,000 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Level 3 fair value measurement, January 1
 
56,465,000 
56,465,000 
59,877,000 
 
Net realized and unrealized gains (losses)
 
 
4,518,000 
2,188,000 
 
Settlements
 
 
(15,843,000)
(5,600,000)
 
Level 3 fair value measurement, December 31
45,140,000 
 
45,140,000 
56,465,000 
 
Pension Plan [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, beginning of period
 
60,856,000 
60,856,000 
 
 
Fair value of plan assets, end of period
57,423,000 
 
57,423,000 
60,856,000 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Level 3 fair value measurement, January 1
 
57,972,000 
57,972,000 
57,914,000 
 
Net realized and unrealized gains (losses)
 
 
(620,000)
58,000 
 
Settlements
 
 
0 
0 
 
Level 3 fair value measurement, December 31
57,352,000 
 
57,352,000 
57,972,000 
 
Pension Plan [Member] |
Level 1 [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
385,441,000 
 
385,441,000 
414,248,000 
 
Pension Plan [Member] |
Level 1 [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
371,655,000 
 
371,655,000 
405,935,000 
 
Pension Plan [Member] |
Level 1 [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 1 [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 1 [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
13,786,000 
 
13,786,000 
5,429,000 
 
Pension Plan [Member] |
Level 1 [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 1 [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
2,884,000 
 
Pension Plan [Member] |
Level 2 [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
251,653,000 
 
251,653,000 
256,476,000 
 
Pension Plan [Member] |
Level 2 [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
58,987,000 
 
58,987,000 
62,150,000 
 
Pension Plan [Member] |
Level 2 [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
47,445,000 
 
47,445,000 
44,651,000 
 
Pension Plan [Member] |
Level 2 [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
129,036,000 
 
129,036,000 
139,396,000 
 
Pension Plan [Member] |
Level 2 [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
16,114,000 
 
16,114,000 
10,279,000 
 
Pension Plan [Member] |
Level 2 [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 2 [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
71,000 
 
71,000 
0 
 
Pension Plan [Member] |
Level 3 [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
102,492,000 
 
102,492,000 
114,437,000 
 
Pension Plan [Member] |
Level 3 [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 3 [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 3 [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 3 [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Pension Plan [Member] |
Level 3 [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
45,140,000 
 
45,140,000 
56,465,000 
 
Pension Plan [Member] |
Level 3 [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
57,352,000 
 
57,352,000 
57,972,000 
 
Other Postretirement Benefit Plan [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Net periodic benefit cost
 
 
2,593,000 
5,033,000 
5,889,000 
Minimum number of years of service for certain employees to be eligible to participate in shared welfare plans that provide postemployment medical and life insurance benefits
 
 
5 
 
 
Plan amendment
 
 
483,000 
11,901,000 
 
Discount rate
4.20% 
 
4.20% 
4.75% 
 
Weighted average discount rate
3.75% 
4.75% 
 
 
5.00% 
Expected long-term return on plan assets
7.75% 
8.00% 
 
7.75% 
7.75% 
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
 
 
 
 
 
Benefit obligation, beginning of period
 
228,253,000 
228,253,000 
257,688,000 
 
Service cost
 
 
2,675,000 
3,257,000 
3,468,000 
Interest cost
 
 
10,235,000 
10,628,000 
11,605,000 
Plan participants' contributions
 
 
3,043,000 
2,915,000 
 
Actuarial loss (gain)
 
 
14,309,000 
(19,702,000)
 
Plan amendment
 
 
483,000 
11,901,000 
 
Benefits paid
 
 
(15,450,000)
(14,632,000)
 
Settlements
 
 
0 
0 
 
Benefit obligation, end of period
243,548,000 
 
243,548,000 
228,253,000 
257,688,000 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, beginning of period
 
155,495,000 
155,495,000 
151,777,000 
 
Actual return on plan assets
 
 
9,733,000 
1,335,000 
 
Employer contributions
 
 
13,225,000 
14,100,000 
 
Plan participants' contributions
 
 
3,043,000 
2,915,000 
 
Benefits paid
 
 
(15,450,000)
(14,632,000)
 
Settlements
 
 
0 
0 
 
Fair value of plan assets, end of period
166,046,000 
 
166,046,000 
155,495,000 
151,777,000 
Balance at December 31
(77,502,000)
 
(77,502,000)
(72,758,000)
 
Current liabilities
0 
 
0 
0 
 
Noncurrent liabilities
(77,502,000)
 
(77,502,000)
(72,758,000)
 
Balance at December 31
(77,502,000)
 
(77,502,000)
(72,758,000)
 
Benefits paid
 
 
(15,450,000)
(14,632,000)
 
Service cost
 
 
2,675,000 
3,257,000 
3,468,000 
Interest cost
 
 
10,235,000 
10,628,000 
11,605,000 
Expected return on assets
 
 
(12,370,000)
(11,892,000)
(11,393,000)
Amortization of unrecognized prior service cost
 
 
(3,316,000)
(2,228,000)
(1,760,000)
Amortization of loss
 
 
5,369,000 
5,268,000 
3,969,000 
Net periodic benefit cost
 
 
2,593,000 
5,033,000 
5,889,000 
Amortization of loss
 
 
 
0 
 
Prior service credit (cost)
10,211,000 
 
10,211,000 
14,010,000 
 
Accumulated loss
(62,084,000)
 
(62,084,000)
(50,447,000)
 
Accumulated other comprehensive loss before regulatory assets
(51,873,000)
 
(51,873,000)
(36,437,000)
 
Regulatory asset for regulated entities
51,873,000 
 
51,873,000 
36,437,000 
 
Accumulated other comprehensive loss after regulatory assets
0 
 
0 
0 
 
Deferred income taxes
0 
 
0 
0 
 
Accumulated other comprehensive loss, net of tax
0 
 
0 
0 
 
Prior service credit (cost) expected to be recognized in following fiscal year
(4,597,000)
 
(4,597,000)
 
 
Actuarial net loss, to be recognized in 2017
6,484,000 
 
6,484,000 
 
 
Effect of a one percentage point change on total of service and interest cost - decrease
 
 
(232,000)
 
 
Effect of a one percentage point change on other postemployment benefit obligation - decrease
 
 
(3,991,000)
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Employer contributions
 
 
13,225,000 
14,100,000 
 
Expected employer contributions in 2017
 
 
3,100,000 
 
 
2017
16,165,000 
 
16,165,000 
 
 
2018
16,815,000 
 
16,815,000 
 
 
2019
17,073,000 
 
17,073,000 
 
 
2020
17,379,000 
 
17,379,000 
 
 
2021
17,401,000 
 
17,401,000 
 
 
2022 through 2027
86,559,000 
 
86,559,000 
 
 
Other Postretirement Benefit Plan [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
47,140,000 
 
47,140,000 
62,058,000 
 
Other Postretirement Benefit Plan [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
75,000 
 
75,000 
64,000 
 
Other Postretirement Benefit Plan [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
19,948,000 
 
19,948,000 
200,000 
 
Other Postretirement Benefit Plan [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
17,063,000 
 
17,063,000 
13,555,000 
 
Other Postretirement Benefit Plan [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
81,820,000 
 
81,820,000 
79,531,000 
 
Other Postretirement Benefit Plan [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
 
 
 
87,000 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
39,891,000 
 
39,891,000 
54,797,000 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
39,817,000 
 
39,817,000 
54,560,000 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
74,000 
 
74,000 
233,000 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 1 [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
 
 
 
4,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
126,155,000 
 
126,155,000 
100,615,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
7,323,000 
 
7,323,000 
7,498,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
75,000 
 
75,000 
64,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
19,948,000 
 
19,948,000 
200,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
16,989,000 
 
16,989,000 
13,322,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
81,820,000 
 
81,820,000 
79,531,000 
 
Other Postretirement Benefit Plan [Member] |
Level 2 [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
 
 
 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
83,000 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member] |
Equity Securities [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member] |
Government Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member] |
Corporate Obligations [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member] |
Cash and Money Market Funds [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member] |
Insurance contracts and group annuity contracts [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
0 
 
0 
0 
 
Other Postretirement Benefit Plan [Member] |
Level 3 [Member] |
Other Investments [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Fair value of plan assets, end of period
 
 
 
83,000 
 
Maximum [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Health care cost-trend rate assumed for next year
 
 
 
7.50% 
 
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)
 
 
 
5.00% 
 
Maximum [Member] |
Pension Plan [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Weighted average discount rate
 
 
 
4.75% 
 
Maximum [Member] |
Other Postretirement Benefit Plan [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Weighted average discount rate
 
 
 
4.75% 
 
Minimum [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
 
 
 
 
 
Health care cost-trend rate assumed for next year
 
 
 
4.00% 
 
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)
 
 
 
4.00% 
 
Minimum [Member] |
Pension Plan [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Weighted average discount rate
 
 
 
4.25% 
 
Minimum [Member] |
Other Postretirement Benefit Plan [Member]
 
 
 
 
 
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
 
 
 
 
 
Weighted average discount rate
 
 
 
4.25% 
 
ONE Gas 401(k) Plan [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Percent of employee contributions matched of eligible compensation (in hundredths)
 
 
100.00% 
 
 
Maximum percentage of each participants eligible compensation subject to certain limits matching (in hundredths)
 
 
6.00% 
 
 
Contributions made to 401(k) plan
 
 
10,800,000 
10,200,000 
 
ONEOK 401(k) Plan [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Expense charged to us by ONEOK, Inc.
 
 
 
 
9,700,000 
ONE Gas Profit-Sharing Plan [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Profit sharing contribution percentage
 
 
1.00% 
 
 
Contributions made to profit-sharing plan
 
 
6,000,000 
6,500,000 
 
ONEOK Profit-Sharing Plan [Member]
 
 
 
 
 
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
 
 
 
 
 
Expense charged to us by ONEOK, Inc.
 
 
 
 
$ 4,000,000 
INCOME TAXES (Details) (USD $)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Deferred Tax Assets, Operating Loss Carryforwards, Domestic
$ 63,000,000 
 
 
Operating Loss Carryforwards, Tax Benefit
11,000,000 
 
 
Deferred Tax Assets, Operating Loss Carryforwards, State and Local
21,000,000 
 
 
Income taxes receivable
1,397,000 
38,877,000 
 
Current income tax provision (benefit)
 
 
 
Federal
(2,016,000)
7,135,000 
17,006,000 
State
471,000 
2,055,000 
1,397,000 
Total current income tax provision (benefit)
(1,545,000)
9,190,000 
18,403,000 
Deferred income tax provision
 
 
 
Federal
76,247,000 
56,440,000 
42,024,000 
State
10,541,000 
7,349,000 
7,911,000 
Total deferred income tax provision
86,788,000 
63,789,000 
49,935,000 
Income Tax Reconciliation [Abstract]
 
 
 
Income before income taxes
225,338,000 
192,009,000 
178,128,000 
Federal statutory income tax rate
35.00% 
35.00% 
35.00% 
Provision for federal income taxes
78,868,000 
67,203,000 
62,345,000 
State income taxes, net of federal benefit
7,158,000 
6,114,000 
6,051,000 
Other, net
(783,000)
(338,000)
(58,000)
Income tax provision
85,243,000 
72,979,000 
68,338,000 
Deferred tax assets
 
 
 
Deferred tax assets, employee benefits and other accrued liabilities
123,333,000 
110,148,000 
 
Net operating loss
23,094,000 
0 
 
Other
5,716,000 
7,848,000 
 
Total deferred tax assets
152,143,000 
117,996,000 
 
Deferred tax liabilities
 
 
 
Excess of tax over book depreciation
990,682,000 
897,667,000 
 
Purchased-gas cost adjustment
13,822,000 
3,999,000 
 
Other regulatory assets and liabilities, net
186,207,000 
168,115,000 
 
Total deferred tax liabilities
1,190,711,000 
1,069,781,000 
 
Net deferred tax liabilities
$ 1,038,568,000 
$ 951,785,000 
 
Minimum [Member]
 
 
 
Operating Loss Carryforwards, Expiration Date
Dec. 31, 2024 
 
 
Maximum [Member]
 
 
 
Operating Loss Carryforwards, Expiration Date
Dec. 31, 2036 
 
 
COMMITMENTS AND CONTINGENCIES - Part 1 (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Operating Leased Assets [Line Items]
 
 
 
Operating Leases, Rent Expense
$ 8.6 
$ 5.0 
$ 5.0 
Operating Leases, Future Minimum Payments Due [Abstract]
 
 
 
2017
5.6 
 
 
2018
5.2 
 
 
2019
4.4 
 
 
2020
3.6 
 
 
2021
3.2 
 
 
Thereafter
4.4 
 
 
Total
$ 26.4 
 
 
COMMITMENTS AND CONTINGENCIES - Part 2 (Details)
12 Months Ended
Dec. 31, 2016
Commitments and Contingencies [Line Items]
 
Number Of Former Manufactured Gas Sites Where We Own Or Retain Legal Responsibility For Environmental Conditions
12 
Number of sites with ongoing groundwater monitoring
8 
Number Of Sites Soil Remediation Is Completed Or Near Completion
11 
Number of sites where regulatory closure has been achieved
3 
Environmental Reserve Estimate Range, Low
4 
Environmental Reserve Estimate Range, High
7 
Environmental Reserve Estimate, Actual
4 
Percentage yield of high consequence pipeline areas
30.00% 
QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Schedule of Quarterly Financial Information [Line Items]
 
 
 
 
 
 
 
 
 
 
 
Revenues
$ 440,754 
$ 232,191 
$ 245,923 
$ 508,364 
$ 389,149 
$ 225,226 
$ 256,786 
$ 676,531 
$ 1,427,232 
$ 1,547,692 
$ 1,818,906 
Net margin
 
 
 
 
 
 
 
 
885,435 
841,733 
826,957 
Operating income
78,534 
30,892 
43,621 
116,073 
73,903 
24,951 
31,270 
109,005 
269,120 
239,129 
225,294 
Net income
$ 42,315 
$ 12,737 
$ 20,300 
$ 64,743 
$ 39,202 
$ 7,371 
$ 12,076 
$ 60,381 
$ 140,095 
$ 119,030 
$ 109,790 
Basic
$ 0.81 
$ 0.24 
$ 0.39 
$ 1.23 
$ 0.75 
$ 0.14 
$ 0.23 
$ 1.15 
$ 2.67 
$ 2.26 
$ 2.10 
Diluted
$ 0.80 
$ 0.24 
$ 0.38 
$ 1.22 
$ 0.74 
$ 0.14 
$ 0.23 
$ 1.13 
$ 2.65 
$ 2.24 
$ 2.07