Document and Entity Information - USD ($) shares in Millions, $ in Billions	12 Months Ended
	Dec. 31, 2019	Feb. 20, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-35636
Entity Registrant Name	ASGN Inc
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	95-4023433
Entity Address, Address Line One	26745 Malibu Hills Road
Entity Address, City or Town	Calabasas
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	91301
City Area Code	818
Local Phone Number	878-7900
Title of 12(b) Security	Common Stock
Trading Symbol	ASGN
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 3.1
Entity Common Stock, Shares Outstanding (in shares)		53.0
Documents Incorporated by Reference	We are incorporating by reference into Part III of this Annual Report on Form 10-K portions of the registrant’s definitive proxy statement for the 2020 Annual Meeting of Stockholders, to be filed within 120 days of the close of the registrant’s fiscal year 2019.
Entity Central Index Key	0000890564
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Current Assets:
Cash and Cash Equivalents	$ 95.2	$ 41.8
Accounts receivable, net	648.7	613.8
Prepaid expenses and income taxes	29.4	11.4
Workers' compensation receivable	13.8	15.0
Other current assets	4.4	4.3
Total current assets	791.5	686.3
Property and equipment, net	73.7	79.1
Operating lease right of use assets	94.6	0.0
Identifiable intangible assets, net	476.5	488.7
Goodwill	1,486.9	1,421.1
Other non-current assets	18.2	12.6
Total assets	2,941.4	2,687.8
Current liabilities:
Accounts payable	39.2	43.1
Accrued payroll and contract professional pay	203.2	194.8
Workers' compensation loss reserves	16.2	17.4
Operating lease liabilities, current	25.8	0.0
Income taxes payable	1.2	3.4
Other current liabilities	55.3	49.5
Total current liabilities	340.9	308.2
Long-term debt	1,032.3	1,100.4
Operating lease liabilities, noncurrent	75.7	0.0
Deferred income tax liabilities	98.7	79.8
Other long-term liabilities	17.6	17.3
Total liabilities	1,565.2	1,505.7
Commitments and contingencies (Note 8)
Stockholders’ equity:
Preferred Stock, Value, Issued	0.0	0.0
Common Stock, Value, Issued	0.5	0.5
Paid-in capital	638.0	601.8
Retained earnings	744.7	586.1
Accumulated other comprehensive loss	(7.0)	(6.3)
Total stockholders’ equity	1,376.2	1,182.1
Total liabilities and stockholders’ equity	$ 2,941.4	$ 2,687.8

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares shares in Millions	Dec. 31, 2019	Dec. 31, 2018
Stockholders’ equity:
Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred Stock, shares authorized (in shares)	1.0	1.0
Preferred Stock, shares issued (in shares)	0.0	0.0
Preferred Stock, shares outstanding (in shares)	0.0	0.0
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (in shares)	75.0	75.0
Common Stock, shares issued (in shares)	52.9	52.5
Common Stock: shares outstanding (in shares)	52.9	52.5

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenues	$ 3,923.9	$ 3,399.8	$ 2,626.0
Cost of services	2,793.9	2,376.1	1,775.9
Gross profit	1,130.0	1,023.7	850.1
Selling, general and administrative expenses	770.3	705.0	592.0
Amortization of intangible assets	51.1	58.5	33.4
Operating income	308.6	260.2	224.7
Interest expense	(52.9)	(56.0)	(27.6)
Write-off of loan costs	(18.9)	0.0	0.0
Income before income taxes	236.8	204.2	197.1
Provision for income taxes	62.0	46.2	39.2
Income from continuing operations	174.8	158.0	157.9
Loss from discontinued operations, net of income taxes	(0.1)	(0.3)	(0.2)
Net income	$ 174.7	$ 157.7	$ 157.7
Earnings per share:
Basic	$ 3.31	$ 3.02	$ 3.01
Diluted	$ 3.28	$ 2.98	$ 2.97
Number of shares and share equivalents used to calculate earnings per share:
Basic (in shares)	52.8	52.3	52.5
Diluted (in shares)	53.4	53.1	53.2
Reconciliation of net income to comprehensive income:
Net income	$ 174.7	$ 157.7	$ 157.7
Foreign currency translation adjustment	(0.7)	(2.7)	6.5
Comprehensive income	$ 174.0	$ 155.0	$ 164.2

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance (in shares) at Dec. 31, 2016		52.7
Balance at Dec. 31, 2016	$ 868.9	$ 0.5	$ 562.9	$ 315.6	$ (10.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		0.4
Vesting of restricted stock units and restricted stock awards	(15.8)	$ 0.0	(15.8)
Employee stock purchase plan (in shares)		0.2
Employee stock purchase plan	7.4	$ 0.0	7.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0.0
Exercise of common stock options	0.4	$ 0.0	0.4
Stock-based compensation expense	24.4		24.4
Stock Repurchased and Retired During Period, Shares		(1.1)
Stock Repurchased and Retired During Period, Value	(58.1)	$ 0.0	(13.2)	(44.9)
Translation adjustments	6.5				6.5
Net income	157.7			157.7
Balance (in shares) at Dec. 31, 2017		52.2
Balance at Dec. 31, 2017	991.4	$ 0.5	566.1	428.4	(3.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		0.1
Vesting of restricted stock units and restricted stock awards	(4.7)	$ 0.0	(4.7)
Employee stock purchase plan (in shares)		0.2
Employee stock purchase plan	8.8	$ 0.0	8.8
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0.0
Exercise of common stock options	0.6		0.6
Stock-based compensation expense	31.0		31.0
Stock Repurchased and Retired During Period, Shares		0.0
Translation adjustments	(2.7)				(2.7)
Net income	$ 157.7			157.7
Balance (in shares) at Dec. 31, 2018	52.5	52.5
Balance at Dec. 31, 2018	$ 1,182.1	$ 0.5	601.8	586.1	(6.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		0.4
Vesting of restricted stock units and restricted stock awards	(12.1)	$ 0.0	(12.1)
Employee stock purchase plan (in shares)		0.2
Employee stock purchase plan	12.6	$ 0.0	12.6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0.1
Exercise of common stock options	0.1		0.1
Stock-based compensation expense	39.5		39.5
Stock Repurchased and Retired During Period, Shares		(0.3)
Stock Repurchased and Retired During Period, Value	(20.0)	$ 0.0	(3.9)	(16.1)
Translation adjustments	(0.7)				(0.7)
Net income	$ 174.7			174.7
Balance (in shares) at Dec. 31, 2019	52.9	52.9
Balance at Dec. 31, 2019	$ 1,376.2	$ 0.5	$ 638.0	$ 744.7	$ (7.0)

CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Millions	12 Months Ended
CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Millions	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Cash Flows from Operating Activities:
Net income	$ 174.7	$ 157.7	$ 157.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	91.2	95.0	58.6
Provision for accounts receivable allowances	3.7	3.3	12.1
Provision (benefit) for deferred income taxes	18.9	11.2	(5.1)
Share-based compensation	39.3	31.5	24.0
Write-off of loan costs	18.9	0.0	0.0
Workers' Compensation provision	3.3	3.6	2.9
Other	(9.0)	(11.7)	(6.6)
Changes in operating assets and liabilities, net of effects of acquisitions:
Accounts receivable	(24.3)	(90.7)	(50.5)
Prepaid expenses and income taxes	20.8	(14.8)	11.9
Income taxes payable	(2.2)	2.1	0.6
Accounts payable	(7.3)	23.3	0.3
Accrued payroll and contract professional pay	5.0	25.4	1.0
Workers' compensation loss reserve	(3.2)	(3.3)	(2.6)
Increase Decrease In Operating Right of Use Asset	28.0	0.0	0.0
Increase (Decrease) In Operating Lease Liability	26.7	0.0	0.0
Other	5.7	1.8	2.7
Net cash provided by operating activities	313.2	287.4	196.4
Cash Flows from Investing Activities:
Cash paid for property and equipment	(32.7)	(28.7)	(24.3)
Cash paid for acquisitions, net of cash acquired	(116.4)	(760.2)	(25.9)
Other	0.0	0.2	0.1
Net cash used in investing activities	(149.1)	(788.7)	(50.1)
Cash Flows from Financing Activities:
Principal payments of long-term debt	(736.2)	(286.0)	(105.0)
Proceeds from long-term debt	653.0	822.0	37.0
Proceeds from option exercises and employee stock purchase plan	12.7	9.4	7.8
Payment of employment taxes related to release of restricted stock awards	(12.2)	(5.6)	(14.9)
Repurchase of common stock	(20.0)	0.0	(60.1)
Debt issuance or amendment costs	(7.8)	(22.5)	(3.3)
Other	0.0	(9.5)	0.0
Net cash provided by (used in) financing activities	(110.5)	507.8	(138.5)
Effect of exchange rate changes on cash and cash equivalents	(0.2)	(1.4)	1.8
Net Increase in Cash and Cash Equivalents	53.4	5.1	9.6
Cash and Cash Equivalents	95.2	41.8	36.7
Supplemental Disclosure of Cash Flow Information
Income taxes	56.6	21.4	55.3
Interest	44.9	51.0	24.1
Non-Cash Investing and Financing Activities:
Unpaid portion of additions to property and equipment	2.2	2.5	2.3
Unpaid portion of debt issuance or amendment costs	$ 1.3	$ 0.0	$ 0.0

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of Consolidation. The consolidated financial statements include the accounts of ASGN Incorporated and its wholly-owned subsidiaries (the “Company”). All intercompany accounts and transactions have been eliminated. Use of Estimates. The preparation of financial statements in conformity with generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Revenue Recognition. Revenues are recognized as control of the promised service is transferred to customers, in an amount that reflects the consideration expected in exchange for the services. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods, see Note 11 – Revenues. Costs of Services. Costs of services include direct costs of contract assignments consisting primarily of payroll, payroll taxes and benefit costs for the Company’s contract professionals. Costs of services also include other direct costs and reimbursable out-of-pocket expenses. Income Taxes. Income taxes are accounted for using the liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that a portion of the deferred tax asset will not be realized. The Company reviews its uncertain tax positions regularly. An uncertain tax position represents the Company’s expected treatment of a tax position taken in a filed return, or planned to be taken in a future tax return or claim that has not been reflected in measuring income tax expense for financial reporting purposes. The Company recognizes the tax benefit from an uncertain tax position when it is more-likely-than-not that the position will be sustained upon examination on the basis of the technical merits or the statute of limitations for the relevant taxing authority to examine and challenge the tax position has expired. Foreign Currency Translation. The functional currency of the Company’s foreign operations is their local currency, and as such, their assets and liabilities are translated into U.S. dollars at the rate of exchange in effect on the balance sheet date. Revenues and expenses are translated at the average rates of exchange prevailing during each monthly period. The related translation adjustments are recorded as cumulative foreign currency translation adjustments in accumulated other comprehensive income (loss) as a separate component of stockholders’ equity. Gains and losses resulting from foreign currency transactions, which are not material, are included in selling, general and administrative (“SG&A”) expenses in the consolidated statements of operations and comprehensive income. Cash and Cash Equivalents. The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents. Accounts Receivable Allowances. The Company estimates an allowance for expected credit losses (the inability of customers to make required payments). These estimates are based on a combination of past experience and current trends. In estimating the allowance for expected credit losses, consideration is given to the current aging of receivables and a specific review for potential bad debts. The resulting bad debt expense is included in SG&A expenses in the consolidated statements of operations and comprehensive income. Receivables are written-off when deemed uncollectible. Leases. Effective January 1, 2019, operating leases are included in operating lease right of use assets and operating lease liabilities on the consolidated balance sheets, see Note 2 – Accounting Standards Update. Property and Equipment. Property and equipment are stated at cost. Depreciation and amortization are provided using the straight-line method over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized over the shorter of the life of the related asset or the remaining term of the lease. Costs associated with customized internal-use software systems that have reached the application development stage and meet recoverability tests are capitalized and include external direct costs utilized in developing or obtaining the applications and payroll and payroll-related expenses for employees who are directly associated with the application development. Business Combinations and Related Acquired Intangible Assets and Goodwill. The Company records all tangible and intangible assets acquired and liabilities assumed in a business combination at fair value as of the acquisition date in accordance with Accounting Standards Codification ("ASC") 805 Business Combinations. Acquisition date fair value represents the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants as measured on the acquisition date. The valuations are based on information that existed as of the acquisition date. During the measurement period, which shall not exceed one year from the acquisition date, provisional amounts may be adjusted to reflect new information the Company has subsequently obtained regarding facts and circumstances that existed as of the acquisition date. Such fair value assessments require judgments and estimates, which may cause final amounts to differ materially from original estimates. The excess amount of the aggregated purchase consideration paid over the fair value of the net of assets acquired and liabilities assumed is recorded as goodwill. Goodwill is evaluated for impairment annually or more frequently if an event occurs or circumstances change, such as material deterioration in performance that would indicate an impairment may exist. The Company performs the annual impairment assessment as of October 31^stfor each of its four reporting units. When evaluating goodwill for impairment, the Company may first perform a qualitative assessment to determine whether it is more likely than not that a reporting unit is impaired. If the Company decides not to perform a qualitative assessment, or if it determines that it is more likely than not that the carrying amount of a reporting unit exceeds its fair value, then the Company performs a quantitative assessment and calculates the estimated fair value of the reporting unit. If the carrying amount of the reporting unit exceeds the estimated fair value, an impairment charge would be recorded to reduce the carrying amount to its estimated fair value. The decision to perform a qualitative impairment assessment in a given year is influenced by a number of factors, including the significance of the excess of the reporting units’ estimated fair value over carrying amount at the last quantitative assessment date, the amount of time in between quantitative fair value assessments and the date of the acquisitions. A qualitative assessment is performed for trademarks to determine if there are any indicators that the carrying amount might not be recovered. A quantitative analysis may be performed in order to test the trademarks for impairment. If a quantitative analysis is necessary, an income approach, specifically a relief from royalty method, is used to estimate the fair value of the trademarks. Principal factors used in the relief from royalty method that require judgment are projected net sales, discount rates, royalty rates and terminal growth assumptions. The estimated fair value of each trademark is compared to its carrying amount to determine if impairment exists. If the carrying amount of a trademark exceeds the estimated fair value, an impairment charge would be recorded to reduce the carrying amount of the trademark. Finite-lived intangible assets are amortized over their useful lives and are tested for recoverability whenever events or changes in circumstances indicate that the carrying amount may not be recoverable. Customer and contractual relationships, backlog and favorable contracts are amortized based on the annual cash flows observed in the valuation of the asset, which generally accelerates the amortization into the earlier years reflective of the economic life of the asset. Contractor relationships, non-compete agreements and in-use software are amortized using the straight-line method. Impairment or Disposal of Long-Lived Assets. The Company evaluates long-lived assets, other than goodwill and identifiable intangible assets with indefinite lives, for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. An impairment loss is recognized when the sum of the undiscounted future cash flows is less than the carrying amount of the asset, in which case a write-down is recorded to reduce the related asset to its estimated fair value. There were no significant impairments of long-lived assets in 2019, 2018 and 2017. Workers’ Compensation Loss Reserves. The Company carries retention policies for its workers’ compensation liability exposures. Under these policies, the Company pays a base premium plus actual losses incurred, not to exceed certain stop-loss limits. The Company is insured for losses above these limits. The Company estimates its workers' compensation loss reserves based on a third- party actuarial study based on claims filed and claims incurred but not reported. The Company accounts for claims incurred but not yet reported based on estimates derived from historical claims experience and current trends of industry data. Changes in estimates and differences in estimates and actual payments for claims are recognized in the period that the estimates changed or the payments were made. Contingencies. The Company records an estimated loss from a loss contingency when information available prior to issuance of its financial statements indicates it is probable that an asset has been impaired or a liability has been incurred at the date of the financial statements and the amount of the loss can be reasonably estimated. Accounting for contingencies, such as legal settlements and workers’ compensation matters, requires the Company to use judgment. Stock-Based Compensation. The Company records compensation expense for restricted stock awards and restricted stock units based on the fair market value of the awards on the date of grant. The fair value of stock-based compensation awards less the estimated forfeitures is amortized over the vesting period of the award. Compensation expense for performance-based awards is measured based on the amount of shares ultimately expected to vest, estimated at each reporting date based on management’s expectations regarding the relevant performance criteria. The Company accounts for employee stock purchase plan shares based on an estimated fair market value using a Black-Scholes valuation model. This methodology requires the use of subjective assumptions such as expected stock price volatility. Concentration of Credit Risk. Financial instruments that potentially subject the Company to credit risks consist primarily of cash and cash equivalents and trade receivables. The Company places its cash and cash equivalents with high quality financial institutions. Concentration of credit risk with respect to accounts receivable for the Apex and Oxford segments is limited because of the large number of clients and their dispersion across different industries and geographies, thus spreading the trade credit risk. The Company performs ongoing credit evaluations to identify risks and maintains an allowance to address these risks. Accounts receivables from the ECS segment are primarily from the U.S. government and are considered to have low credit risk.

Accounting Standards Update	12 Months Ended
Accounting Standards Update	Dec. 31, 2019
Accounting Changes and Error Corrections [Abstract]
Accounting Standards Update	Accounting Standards Update Effective January 1, 2019, the Company adopted ASU 2016-02 Leases (Topic 842), which requires lessees to recognize most operating leases on the balance sheet as a right of use asset and lease liability. The Company recognized and measured the right of use asset and lease liability from operating leases on the consolidated balance sheet without revising comparative period information or disclosures, see Note 3 – Leases. Effective January 1, 2019, the Company adopted SEC Release No. 33-10532, Disclosure Update and Simplification, amending certain disclosure requirements that were redundant, duplicative, overlapping, outdated or superseded. In addition, the amendments expanded the disclosure requirements on the analysis of stockholders' equity for interim financial statements. The adoption of this guidance did not have a material impact on the Company's consolidated financial statements. Effective January 1, 2020, the Company adopted ASU No. 2016-13 Financial Instruments Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments. This guidance requires a financial asset (or a group of financial assets) that is measured at amortized cost basis to be presented at the net amount expected to be collected. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The Company applied this guidance to its methodology for estimating the accounts receivable allowance for doubtful accounts. The adoption of this guidance will not have a material impact on the Company's consolidated financial statements. Effective January 1, 2020, the Company adopted ASU No. 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The amendments in this update are effective for interim and annual periods and were adopted prospectively. The adoption of this guidance will not have a material impact on the Company's consolidated financial statements. Effective January 1, 2020, the Company adopted ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350) Simplifying the Test for Goodwill Impairment. The new guidance eliminates Step 2 of the goodwill impairment test, which requires companies to calculate the implied fair value of goodwill, determined in the same manner as the amount of goodwill recognized in a business combination and using a hypothetical purchase price allocation. Under the new guidance, goodwill impairment will now be measured as the amount by which a reporting unit’s carrying amount exceeds its fair value. The adoption of this guidance will not have a material impact on the Company's consolidated financial statements. In December 2019, the Financial Accounting Standards Board ("FASB") issued ASU No. 2019-12, Income Taxes (Topic 740), which removes certain exceptions to the general principles in Topic 740. The amendments in this update also improve consistent application of and simplify GAAP for other areas of Topic 740 by clarifying and amending existing guidance. The amendments in this update are effective for interim and annual periods for the Company beginning on January 1, 2021, with early adoption permitted. The Company is in the process of evaluating the impact the standard will have on its consolidated financial statements.

Leases (Notes)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Lease Disclosure

Leases

Effective January 1, 2019, the Company adopted Accounting Standards Update ("ASU") 2016-02 Leases (Accounting Standards Codification Topic "ASC" 842), which requires lessees to recognize most operating leases on the balance sheet as a right of use ("ROU") asset and lease liability. The Company adopted this standard using the optional transition method measuring and recognizing the ROU asset and lease liability from operating leases on the condensed consolidated balance sheet without comparative period information or disclosures. The adoption of the standard did not have an effect on the Company’s results of operations, stockholders' equity or cash flows.

The Company elected the package of practical expedients which specifies entities do not need to reassess expired or existing contracts as of the adoption date for the following items: (i) determination of whether a contract is or contains a lease, (ii) revising classification of leases and (iii) assessment of initial direct costs. For existing or expired contracts as of the adoption date, the determinations made for these items under the previous accounting standard (ASC 840) were retained at transition, as allowed by this package of practical expedients.

The Company has operating leases for corporate offices, branch offices and data centers. At the transition date, the operating lease ROU asset and operating lease liability were $93.9 million and $99.4 million, respectively. The difference between the operating lease ROU asset and operating lease liability is due to deferred rent and prepaid rent balances that were reclassified as a component of the ROU asset at the transition date.

The Company's leases have remaining lease terms of one month to eight years. At the inception of a contract, the Company determines if the contract contains a lease. A contract contains a lease if it conveys the right to control the use of an identified asset for a period of time in exchange for consideration. Operating lease ROU assets and operating lease liabilities are recognized at the lease commencement date, based on the present value of the future minimum lease payments. The Company’s leases do not provide an implicit rate of return. Therefore, the Company uses its incremental borrowing rate (“IBR”) in determining the present value of lease payments. In determining the IBR, the Company considers its credit rating and the current market interest rates. The IBR approximates the interest rate the Company would pay on collateralized debt with similar terms and payments as the lease agreements and in a similar economic environment where the leased assets are located. Leases with an initial term of 12 months or less ("short-term leases") are not recorded on the balance sheet. The Company does not have finance leases.

Lease expense is recognized on a straight-line basis over the lease term and is primarily included in selling, general and administrative expenses. Some lease agreements offer renewal options which are assessed against relevant economic factors to determine whether it is reasonably certain that these renewal options will be exercised. As a result of this assessment, for most leases, renewal options were excluded from the minimum lease payments when calculating the operating lease ROU assets and operating lease liabilities, as the Company does not consider the exercise of such options to be reasonably certain.

The Company has lease agreements with lease and non-lease components, which are accounted for as a single lease component for all underlying asset classes. Some leases require variable payments for common area maintenance, property taxes, parking, insurance and other variable costs. The variable portion of lease payments is not included in operating lease ROU assets or operating lease liabilities. Variable lease costs are expensed when incurred.

Components of lease expense for the year ended December 31, 2019 were as follows (in millions):



Operating lease expense	$	32.1
Short-term lease expense	2.1
Variable lease expense	5.8
Total lease expense	$	40.0

Supplemental cash flow information related to leases for December 31, 2019 (in millions):



Cash paid for operating lease liabilities	$	32.1
Operating lease ROU assets obtained in exchange for new operating lease liabilities	$	30.0
Weighted-average remaining lease term of operating leases	4.2 years
Weighted-average discount rate of operating leases	4.26		%

Maturities of operating lease liabilities at December 31, 2019 (in millions):



2020	$	29.5
2021	28.1
2022	22.1
2023	16.1
2024	9.3
Thereafter	5.6
Total future minimum lease payments	110.7
Less imputed interest	9.2
Total operating lease liabilities	$	101.5

The Company leases two properties owned indirectly by certain board members and an executive of the Company. Rent expense for these two properties was $1.2 million for the year 2019 and $1.3 million for each of the years 2018 and 2017. These related party leases are scheduled to end in October 2024.

At December 31, 2019, the Company has additional operating leases, primarily for real estate that have not yet commenced, with total future lease payments of approximately $2.0 million. These operating leases will commence in 2020 with lease terms of approximately 5.3 years.

Disclosures Related to Periods Prior to Adopting the New Lease Standard

Rent expense was $32.7 million in 2018 and $26.4 million in 2017. As previously disclosed in the Company's 2018 Annual Report on Form 10-K under the previous lease accounting standard, minimum rental commitments under non-cancelable operating leases in effect at December 31, 2018 were (in millions):



	Operating Leases		Related Party Leases		Total
2019	$	29.6	$	1.3	$	30.9
2020	24.1		1.3		25.4
2021	19.8		1.4		21.2
2022	14.2		1.4		15.6
2023	8.9		1.4		10.3
Thereafter	6.3		1.1		7.4
Total	$	102.9	$	7.9	$	110.8

Acquisitions

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Acquisitions

Intersys Acquisition

On October 17, 2019, the Company acquired all of the membership interests of Intersys Consulting, LLC ("Intersys"), headquartered in Austin, Texas, for $67.0 million in cash. The acquisition expands the Company's capabilities in digital innovation and enterprise solutions and it is part of the Apex Segment. Goodwill associated with this acquisition totaled $41.4 million, of which $38.7 million is deductible for income tax purposes. Identifiable intangible assets related to this acquisition totaled $23.8 million and the weighted-average amortization period for identifiable intangible assets is approximately 9 years. The results of operations of Intersys are included in the consolidated results of the Company from the date of its acquisition. The purchase accounting for the acquisition of Intersys remains incomplete with respect to the provisional fair value of assets acquired and liabilities assumed, as management continues to gather and evaluate information about circumstances that existed as of the acquisition date. Measurement period adjustments will be recognized prospectively within 12 months from the date of acquisition.

DHA Acquisition

On January 25, 2019, the Company acquired all of the outstanding shares of DHA Group, Inc. ("DHA"), headquartered in Washington, D.C., for $46.0 million in cash. DHA is a provider of mobility, cybersecurity, cloud and IT services to the Federal Bureau of Investigation and other federal customers. DHA was acquired to expand the Company's government services and is part of the ECS segment. Goodwill associated with this acquisition totaled $24.7 million, which is deductible for income tax purposes. Identifiable intangible assets related to this acquisition totaled $19.0 million and the weighted-average amortization period for identifiable intangible assets is approximately 9 years. The results of operations of DHA are included in the consolidated results of the Company from the date of its acquisition. The purchase accounting for this acquisition has been finalized.

ECS Acquisition

On April 2, 2018, the Company acquired all of the outstanding equity interests of ECS Federal, LLC ("ECS") for $775.0 million. Acquisition expenses were approximately $12.0 million and were included in SG&A expenses. ECS, which is headquartered in Fairfax, Virginia, is a leading provider of government IT services and solutions. The ECS acquisition allows the Company to compete in the federal IT and professional services sector. ECS is reported as a separate segment of the Company.

The results of operations of ECS are included in the consolidated results of the Company from the date of its acquisition. The consolidated statements of operations and comprehensive income for the year ended December 31, 2018 included revenues from ECS of $493.0 million and income before income taxes of $14.2 million, respectively.

Assets and liabilities of all acquired companies are recorded at their estimated fair values at the dates of acquisition. The fair value assigned to identifiable intangible assets was primarily determined using a discounted cash flow method, which is a non-recurring fair value measurement based on unobservable inputs (Level 3 inputs). Goodwill represents the acquired assembled workforce, potential new customers and future cash flows after the acquisition. Goodwill related to this acquisition totaled $528.2 million, of which $514.2 million is estimated to be deductible for income tax purposes. Preliminary fair values of assets acquired and liabilities assumed have been updated for working capital adjustments, deferred taxes and completion of the valuation of identifiable intangible assets. The purchase accounting for this acquisition has been finalized.

The following table summarizes the consideration paid and the fair value of assets acquired and liabilities assumed (in millions):



Cash	$	12.4
Accounts receivable	97.2
Prepaid expenses and other current assets	8.6
Property and equipment	29.0
Identifiable intangible assets	195.0
Goodwill	528.2
Other non-current assets	1.2
Total assets acquired	871.6

Current liabilities	94.7
Long-term liabilities	4.3
Total liabilities assumed	99.0

Total purchase price	$	772.6

The following table summarizes the acquired identifiable intangible assets of ECS (in millions):



	Useful life
Contractual customer relationships	12.75 years	$	144.6
Backlog	2.75 years	23.1
Non-compete agreements	4 to 7 years	10.3
Favorable contracts	5 years	0.5
Trademarks	indefinite	16.5
Total identifiable intangible assets acquired		$	195.0

The weighted-average amortization period for identifiable intangible assets, excluding trademark, is 11 years.

The summary below (in millions, except for per share data) presents pro forma unaudited consolidated results of operations for the years ended December 31, 2018 and 2017 as if the acquisition of ECS by the Company and the acquisition of a business by ECS in April 2017, both occurred on January 1, 2017. The pro forma unaudited consolidated results give effect to, among other things: (i) amortization of intangible assets, (ii) stock-based compensation expense and the related dilution for restricted stock units granted to ECS employees, (iii) interest expense on acquisition-related debt and (iv) the exclusion of nonrecurring expenses incurred by ECS prior to its acquisition by the Company for ECS’ acquisition-related activities and costs incurred in the sale of ECS to the Company. The pro forma results do not include pre-acquisition results of DHA or Intersys due to their size. The pro forma unaudited consolidated results are not necessarily indicative of the operating results that would have occurred if the acquisition had been consummated as of the date indicated, nor are they necessarily indicative of future operating results.



	2018		2017
Revenues	$	3,548.9	$	3,213.5
Income from continuing operations	$	169.6	$	134.8
Net income	$	169.3	$	134.6

Earnings per share:
Basic	$	3.24	$	2.57
Diluted	$	3.19	$	2.53

Number of shares and share equivalents used to calculate earnings per share:
Basic	52.4		52.5
Diluted	53.2		53.2

Goodwill and Identifiable Assets

12 Months Ended

Dec. 31, 2019

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Identifiable Intangible Assets

The changes in the carrying amount of goodwill for the years ended December 31, 2019 and 2018 are as follows (in millions):



	Apex Segment		Oxford Segment			ECS Segment		Total
Balance as of December 31, 2017	$	662.1	$	232.0		$	—	$	894.1
ECS acquisition	—		—			528.2		528.2
Translation adjustment	—		(1.2		)	—		(1.2		)
Balance as of December 31, 2018	662.1		230.8			528.2		1,421.1
DHA acquisition	—		—			24.7		24.7
Intersys acquisition	41.4		—			—		41.4
Translation adjustment	—		(0.3		)	—		(0.3		)
Balance as of December 31, 2019	$	703.5	$	230.5		$	552.9	$	1,486.9

Acquired intangible assets consisted of the following (in millions):



		December 31, 2019						December 31, 2018
	Estimated Useful Life	Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount
Subject to amortization:
Customer and contractual relationships	2 - 12.75 years	$	384.9	$	179.9	$	205.0	$	346.9	$	145.4	$	201.5
Contractor relationships	2 - 5 years	71.1		70.6		0.5		71.1		67.1		4.0
Backlog	1 - 2.75 years	25.0		23.9		1.1		23.1		17.7		5.4
Non-compete agreements	2 - 7 years	24.8		13.8		11.0		22.1		9.9		12.2
In-use software	6 years	18.9		18.9		—		18.9		16.0		2.9
Favorable contracts	5 years	—		—		—		1.4		0.9		0.5
		524.7		307.1		217.6		483.5		257.0		226.5
Not subject to amortization:
Trademarks⁽¹⁾		258.9		—		258.9		262.2		—		262.2
Total		$	783.6	$	307.1	$	476.5	$	745.7	$	257.0	$	488.7

⁽¹⁾ Certain foreign trademarks totaling $3.3 million were written off during the second quarter of 2019.

Amortization expense for intangible assets with finite lives was $51.1 million in 2019, $58.5 million in 2018 and $33.4 million in 2017. Estimated amortization for each of the next five years and thereafter follows (in millions):



2020	$	44.7
2021	37.8
2022	29.4
2023	25.4
2024	19.1
Thereafter	61.2
	$	217.6

Property and Equipment

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

Property and Equipment.

Property and Equipment

Property and equipment at December 31, 2019 and 2018 consisted of the following (in millions):



	2019			2018
Computer hardware and software	$	180.2		$	159.4
Furniture, fixtures and equipment	26.8			22.2
Leasehold improvements	24.7			19.2
Work-in-progress	7.0			3.7
	238.7			204.5
Less -- accumulated depreciation	(165.0		)	(125.4		)
	$	73.7		$	79.1

The Company has capitalized costs related to its various technology initiatives. At December 31, 2019 and 2018, the net book value of the property and equipment related to computer software was $34.8 million and $41.3 million, respectively, which included work-in-progress of $6.4 million and $3.1 million, respectively.

Depreciation expense related to property and equipment was $40.1 million in 2019, $36.5 million in 2018 and $25.2 million in 2017 and is included in SG&A expenses and costs of services ($10.3 million and $7.7 million in costs of services in 2019 and 2018, respectively).

Long-Term Debt

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Long-Term Debt

At December 31, 2019 and 2018, long-term debt consisted of the following (in millions):



	2019			2018
Senior Credit Facility:
$250 million revolving credit facility, due November 22, 2024	$	—		$	—
Term B loan facility, due June 5, 2022	—			337.0
Term B loan facility, due April 2, 2025	490.8			787.0
Senior Notes, due May 15, 2028	550.0			—
	1,040.8			1,124.0
Unamortized deferred loan costs	(8.5		)	(23.6		)
	$	1,032.3		$	1,100.4

Senior Credit Facility

On November 22, 2019, the Company entered into the sixth amendment to its senior credit agreement, which provides for, among other things, (i) an increase in the aggregate commitments available under the revolving credit facility to $250.0 million and an extension of its maturity date to November 2024 and (ii) a reduction of 25 basis points in the applicable margin for the term loans. The Company wrote-off deferred loan costs totaling $18.9 million related to repayment (and retirement) of the term B loan facility due 2022 and partial repayment of the outstanding loans under the term B loan facility due 2025.

At December 31, 2019, the interest on the term B loans was 3.55 percent. Borrowings under the revolving credit facility bear interest at LIBOR plus 1.25 to 2.25 percent, or the bank’s base rate plus 0.25 to 1.25 percent, depending on leverage levels. A commitment fee of 0.20 to 0.35 percent is payable on the undrawn portion of the revolving credit facility. There are no required minimum payments for any of the Company's debt instruments until their maturity dates. The Company is required to make mandatory prepayments on its term loans from excess cash flow and with the proceeds of asset sales, debt issuances and specified other events, subject to certain exceptions. The senior credit facility is secured by substantially all of our assets and includes various restrictive covenants including the maximum ratio of consolidated secured debt to consolidated EBITDA, which steps down at regular intervals from 4.50 to 1.00 as of December 31, 2019, to 3.75 to 1.00 as of September 30, 2021. The senior credit facility also contains certain customary limitations including, among other terms and conditions, the Company's ability to incur additional indebtedness, engage in mergers and acquisitions and declare dividends. At December 31, 2019, the Company was in compliance with its debt covenants. At December 31, 2019, the ratio of consolidated debt to consolidated EBITDA was 2.30 to 1.00 and the Company had $246.1 million available borrowing capacity under its revolving credit facility.

Senior Notes

On November 22, 2019, the Company issued $550.0 million of 4.625 percent senior notes due 2028 (the "Senior Notes"). The Company used the proceeds from the Senior Notes to repay or paid down borrowings under its senior credit facility. Interest on the Senior Notes is payable in arrears on May 15 and November 15 of each year beginning on May 15, 2020. The Senior Notes are senior unsecured obligations and are effectively subordinated to the Company’s existing and future secured indebtedness (including the secured indebtedness under the Company's

senior credit agreement) to the extent of the value of the collateral securing that indebtedness and are structurally subordinated to all of the liabilities of any of the Company's subsidiaries that do not guarantee the notes. The Senior Notes also contain certain customary limitations including, among other terms and conditions, the Company's ability to incur additional indebtedness, engage in mergers and acquisitions, transfer or sell assets and make certain distributions.

In connection with the issuance of the Senior Notes and the sixth amendment to the senior credit agreement, the Company incurred $9.1 million of debt issuance and amendment costs, of which $8.6 million are presented in the consolidated balance sheet as a reduction of outstanding debt and are being amortized over the term of the Senior Notes and the term loans and $0.5 million fees were presented in other current assets and other non-current assets and are being amortized over the term of the revolving credit facility.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

The following is a summary of the Company's purchase obligations as of December 31, 2019, excluding lease liabilities (see Note 3 – Leases) and other current liabilities that are included in the consolidated balance sheet (in millions):



	Purchase Obligations
2020	$	13.4
2021	11.6
2022	11.3
2023	4.4
2024	—
Thereafter	—
Total	$	40.7

Purchase obligations are non-cancelable job board service agreements, software maintenance and license agreements and software subscriptions.

The workers' compensation loss reserves were $2.4 million and $2.4 million, net of anticipated insurance and indemnification recoveries of $13.8 million and $15.0 million, at December 31, 2019 and 2018, respectively. The Company has unused stand-by letters of credit outstanding to secure obligations for workers’ compensation claims and other obligations. The unused stand-by letters of credit at December 31, 2019 and 2018 were $3.9 million and $4.4 million, respectively.

Certain employees participate in the Company’s Second Amended and Restated Change in Control Severance Plan and/or have separate agreements that provide for certain benefits in the event of termination at the Company's convenience, as defined by the plan or agreement. Generally, these benefits are based on the employee’s position with the Company and include severance, continuation of health insurance and may contain a pro rata bonus.

Legal Proceedings

The Company is involved in various legal proceedings, claims and litigation arising in the ordinary course of business. The Company does not believe that the disposition of matters that are pending or asserted will have a material effect on its consolidated financial statements.

Stockholders' Equity	12 Months Ended
Stockholders' Equity	Dec. 31, 2019
Equity, Class of Treasury Stock [Line Items]
Treasury Stock [Text Block]	Stockholders' Equity Under a two-year stock repurchase program that expired on June 10, 2018, Company repurchased 1.1 million shares in 2017 at a cost of $58.1 million. All shares repurchased were retired, which resulted in a reduction in paid-in capital of $13.2 million and a reduction in retained earnings of $44.9 million in 2017. There were no repurchases in 2018. On May 31, 2019, the Board of Directors approved a $250.0 million, two-year stock repurchase program. Under this program, the Company repurchased 0.3 million shares of its common stock in 2019 at a cost of $20.0 million. All repurchased shares were retired, which resulted in a reduction in paid-in capital of $3.9 million and a reduction in retained earnings of $16.1 million in 2019. The balances of accumulated other comprehensive income (loss) at December 31, 2019, 2018 and 2017 and the activity within those years was comprised of foreign currency translation adjustments.

Stock-based Compensation and Other Employee Benefit Plans

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan

Stock-Based Compensation and Other Employee Benefit Plans

On June 13, 2019, the stockholders of the Company approved the Second Amended and Restated 2010 Incentive Award Plan (the “2010 Plan”). This plan permits the grant of incentive stock options, nonqualified stock options, dividend equivalent rights, stock payments, deferred stock, restricted stock awards, restricted stock units (“RSUs”), performance shares and other incentive awards, stock appreciation rights and cash awards to its employees, directors and consultants. As of December 31, 2019, there were 2.9 million shares available for issuance under the 2010 Plan.

The Board of Directors adopted the Second Amended and Restated 2012 Employment Inducement Incentive Award Plan on April 26, 2018 (the “2012 Plan”). This plan allows for grants of stock to employees as employment inducement awards pursuant to NYSE rules. The terms of the 2012 Plan are similar to the 2010 Plan. As of December 31, 2019, there were 0.1 million shares available for issuance under the 2012 Plan.

The Company believes that stock-based compensation aligns the interests of its employees and directors with those of its stockholders. Stock-based compensation provides incentives to retain and motivate executive officers and key employees responsible for driving Company performance and maintaining important relationships that contribute to the growth of the Company.

Stock-based compensation expense, which is included in SG&A expenses, was $39.3 million, $31.5 million and $24.0 million for the years ended December 31, 2019, 2018 and 2017, respectively. The Company recognized $1.1 million, $2.7 million and $4.5 million of excess tax benefits from stock-based compensation in 2019, 2018 and 2017, respectively.

Restricted Stock Units

The fair value of each RSU is based on the fair market value of the awards on the grant date and the Company records compensation expense based on this value, net of a forfeiture rate. The forfeiture rate estimates the number of awards that will eventually vest and is based on historical vesting patterns for RSUs.

A summary of the status of the Company’s unvested RSUs as of December 31, 2019 and changes during the year then ended are presented below (number of units in millions, except fair value per unit):



	Service Conditions RSUs		Performance and Service Conditions RSUs		Total RSUs		Weighted Average Grant-Date Fair Value Per Unit
Unvested RSUs outstanding at December 31, 2018	0.4		0.9		1.3			$	60.87
Granted	0.2		0.4		0.6			$	62.26
Vested	(0.1	)	(0.5	)	(0.6	)		$	56.58
Forfeited	—		(0.1	)	(0.1	)		$	67.58
Unvested RSUs outstanding at December 31, 2019	0.5		0.7		1.2			$	63.21
Unvested and expected to vest RSUs outstanding at December 31, 2019	0.4		0.7		1.1			$	62.96

The total number of shares vested in the table above includes 0.2 million shares surrendered by the employees to the Company for payment of employees' income taxes. The surrendered shares are available for issuance under the 2010 Plan.

The weighted-average grant-date fair value of RSUs granted during 2019, 2018 and 2017 was $62.26, $74.61 and $49.62 per unit, respectively. The fair value of RSUs that vested during 2019, 2018 and 2017 was $38.7 million, $16.4 million and $37.8 million, respectively.

As of December 31, 2019, there was unrecognized compensation expense of $38.5 million related to unvested RSUs based on awards that are expected to vest. The unrecognized compensation expense is expected to be recognized over a weighted-average period of two years.

Liability Awards

The Company's liability awards have a performance component and vest in one year from the date of grant. The performance goals are approved by the Compensation Committee of the Company’s Board of Directors. The Company classifies these awards as a liability until the number of shares is determined in accordance with the grant. The number of shares is determined by dividing the final award liability balance by the Company’s closing stock price on the settlement date. The outstanding balances of these liability awards were $0.2 million and $0.5 million at December 31, 2019 and 2018, respectively and were included in other accrued expenses in the accompanying consolidated balance sheets. There was no unrecognized compensation expense for liability awards as of December 31, 2019.

Stock Options

The Company has not granted stock options since 2012. The activity during the year for exercised stock options and outstanding stock options at the end of the year were insignificant.

Employee Stock Purchase Plan

The stockholders of the Company approved the Company’s 2010 Employee Stock Purchase Plan on June 3, 2010, which plan has been amended from time to time (the “ESPP”). The ESPP allows eligible employees to purchase common stock of the Company, through payroll deductions, at a 15 percent discount of the lower of the market price on the first day or the last day of the semi-annual purchase periods. Participants are required to hold the shares for a 12-month period after the purchase date. The ESPP is intended to qualify as an employee stock purchase plan under the Internal Revenue Service (“IRS”) Code Section 423. Eligible employees may contribute up to a certain percentage set by the plan administrator of their eligible earnings toward the purchase of the stock (subject to certain IRS limitations). As of December 31, 2019, there were 1.7 million shares available for issuance under the ESPP.

Shares of common stock are transferred to participating employees at the conclusion of each six-month offering period, which ends on the last business day of the month in March and September each year. Compensation expense is measured using a Black-Scholes valuation model. The table below presents the average Black-Scholes valuation per share of shares purchased and the compensation expense under the ESPP (in millions, except per share amounts):



Year Ended December 31,	Average Black-Scholes Valuation per Share	Shares	Expense
2019	$17.11	0.2	$	4.1
2018	$15.09	0.2	$	2.7
2017	$9.71	0.2	$	2.1

The assumptions used in the Black-Scholes valuation model for shares purchased under the ESPP, for all periods presented, are: expected term of 0.5 years, dividend yield of zero percent, volatility rate in the range of 23.0 percent to 38.5 percent and risk free interest rate in the range of 0.45 percent to 2.37 percent.

Deferred Compensation Plan

The Company’s Deferred Compensation Plan, which became effective on June 1, 2017 and has been amended from time to time (the "DCP"), allows for eligible management and highly compensated key employees to elect to defer a portion of their compensation to later years. These deferrals are immediately vested and are subject to investment risk and a risk of forfeiture under certain circumstances. Participants may choose from various investment options representing a broad range of asset classes. The Company’s deferred compensation plan liability was $11.8 million and $6.2 million at December 31, 2019 and 2018, respectively, which was primarily included in other long-term liabilities. The Company established a rabbi trust to fund the deferred compensation plan (see Note 15 – Fair Value Measurements).

Employee Defined Contribution Plans

The Company maintains various 401(k) retirement savings plans for the benefit of our eligible U.S. employees. Under terms of these plans, eligible employees are able to make contributions to these plans on a tax-deferred basis. The Company makes matching contributions, some of which are discretionary. The Company made contributions to the 401(k) plans of $16.2 million, $13.0 million and $7.5 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Revenues	12 Months Ended
Revenues	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenues Adoption of ASC Topic 606, Revenue from Contracts with Customers (ASC 606) Effective January 1, 2018, the Company adopted ASC 606 using the modified retrospective method, which allows companies to apply the new revenue standard to reporting periods beginning in the year the standard is first implemented, while prior periods continue to be reported in accordance with previous accounting guidance. The adoption of ASC 606 did not have a significant impact on the recognition of revenues; therefore, the Company did not have an opening retained earnings adjustment. Disaggregated revenue disclosures by segment are presented in Note 14 – Business Segments. Revenue Recognition Revenues are recognized as control of the promised service is transferred to customers, in an amount that reflects the consideration expected in exchange for the services. For the Apex and Oxford segments, revenues from assignment contracts are recognized over time, based on hours worked by the Company’s contract professionals. The performance of the requested service over time is the single performance obligation for assignment revenues. Certain customers may receive discounts (e.g., volume discounts, rebates, prompt-pay discounts) and adjustments to the amounts billed. These discounts, rebates and adjustments are considered variable consideration. Volume discounts are the largest component of variable consideration and are estimated using (i) the most likely amount method prescribed by ASC 606, (ii) contract terms and (iii) estimates of revenue. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods. Permanent placement revenues are recognized at the point in time when employment candidates begin permanent employment. Finding a qualified candidate that the client hires as a permanent employee is a single performance obligation for the Company’s permanent placement contracts. Revenues recognized from permanent placement services are based upon a percentage of the candidates' base salary. The Company records an estimated liability for permanent placement candidates that are not expected remain with the client through the contingency period, which is typically 90 days ("fallouts"). When a fallout occurs, the Company will generally find a replacement candidate at no additional cost to the client. Prior to the adoption of ASC 606, the estimate for permanent placement fallouts was recorded as accounts receivable allowances and effective January 1, 2018 this estimate is considered a contract liability and was $1.5 million. On April 2, 2018, the Company acquired ECS, which delivers advanced solutions in cloud, cybersecurity, artificial intelligence, machine learning, software development, IT modernization and science and engineering and is primarily focused on federal government activities (see Note 4 – Acquisitions). ECS customer contracts generally contain a single performance obligation involving a significant integration of various activities that are performed together to deliver a combined service or solution. Performance obligations may involve a series of recurring services, such as network operations and maintenance, operation and program support services, IT outsourcing services and other IT arrangements where the Company is standing ready to provide support, when-and-if needed. Performance obligations are satisfied over time because the customer simultaneously receives and consumes the benefits of the Company’s performance as services are provided. ECS provides services under the following types of contracts: Time and materials ("T&M") contracts provide for payments based on fixed hourly rates for each direct labor hour expended and reimbursements for allowable material costs and out-of-pocket expenses. To the extent actual direct labor and associated costs vary in relation to the agreed upon billing rates, the generated profit may vary. Cost reimbursable contracts provide for reimbursement of direct contract costs and allowable and allocable indirect costs, plus a negotiated profit margin or fee. Cost reimbursable contracts are usually subject to lower risk and tend to have lower margins. Firm-fixed-price ("FFP") contracts provide for a fixed price for specified services and solutions. If actual costs vary from planned costs on an FFP contract, the Company generates more or less than the planned amount of profit. Revenues for T&M contracts are recognized over time, based on hours worked. Revenues for cost reimbursable contracts, under which the Company bills the customer for actual costs incurred plus a negotiated fee and FFP contracts are recognized over time, generally based on the amount invoiced as those amounts directly correspond with the value received by a customer. From time to time, the Company may have FFP contracts in which revenues are recognized using a cost-to-cost measurement method. The Company recognizes revenues on a gross basis as it acts as a principal for all of its revenue transactions. The Company has direct contractual relationships with its customers, bears the risks and rewards of its arrangements, has the discretion to select the contract professionals and establish the price for the services to be provided. The Company includes billable expenses (allowable material costs and out-of-pocket reimbursable expenses) in revenues and the associated expenses are included in costs of services. The Company’s contracts have termination for convenience provisions and do not have substantive termination penalties; therefore, the contract duration for accounting purposes may be less than the stated terms. For accounting purposes, the Company's contracts with customers are considered to be of a short-term nature (one year or less). The Company does not disclose the value of remaining performance obligations for short-term contracts. Payment Terms Payment terms vary and the time between invoicing and when payment is due is not significant. There are no financing components to the Company’s arrangements. Contract Liabilities for Advance Payments The Company has contract liabilities for payments received in advance of providing services under certain contracts. Contract liabilities for advance payments were $8.4 million and $9.8 million at December 31, 2019 and 2018, respectively. Contract liabilities are included in other current liabilities on the consolidated balance sheet and are generally recognized as revenues within three months from the balance sheet date. Contract Costs There are no incremental costs to obtain contracts. Contract fulfillment costs include, but are not limited to, direct labor for both employees and subcontractors, allowable materials such as third-party hardware and software that are integrated as part of the overall services and solutions provided to customers and out-of-pocket reimbursable expenses. Contract fulfillment costs are expensed as incurred, except for certain set-up costs for an ECS project, which were capitalized and are being amortized over the expected period of benefit. Accounts Receivable Allowances The Company estimates its credit losses (the inability of customers to make required payments) based on (i) a combination of past experience and current trends, (ii) consideration of the current aging of receivables and (iii) a specific review for potential bad debts. The resulting bad debt expense is included in selling, general and administrative ("SG&A") expenses. The accounts receivable allowance was $5.1 million and $4.8 million at December 31, 2019 and 2018, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

The provision for income taxes consists of the following (in millions):



	Year Ended December 31,
	2019			2018			2017
Current:
Federal	$	29.9		$	20.6		$	33.6
State	8.6			10.6			7.7
Foreign	4.6			3.9			2.8
	43.1			35.1			44.1
Deferred:
Federal and State	19.9			11.5			(4.7		)
Foreign	(1.0		)	(0.4		)	(0.2		)
	18.9			11.1			(4.9		)
	$	62.0		$	46.2		$	39.2

Income from continuing operations before income taxes consists of the following (in millions):



	Year Ended December 31,
	2019		2018		2017
United States	$	218.7	$	190.7	$	184.2
Foreign	18.1		13.5		12.9
	$	236.8	$	204.2	$	197.1

The components of deferred tax assets (liabilities) are as follows (in millions):



	December 31,
	2019			2018
Intangibles	$	(112.7	)	$	(90.8	)
Depreciation expense	(13.3		)	(15.0		)
Operating lease right of use assets	(24.8		)	—
Operating lease liabilities	25.7			—
Allowance for doubtful accounts	1.8			1.6
Employee-related accruals	12.0			9.6
Stock-based compensation	9.2			9.5
Other	3.4			5.3
Net operating loss carryforwards–foreign	0.8			1.0
Valuation allowance	(0.8		)	(1.0		)
	$	(98.7	)	$	(79.8	)

The reconciliation between the amount computed by applying the U.S. federal statutory tax rate of 21 percent in 2019 and 2018 and 35 percent in 2017 to income before income taxes, for each respective year and the income tax provision is as follows (in millions):



	Year Ended December 31,
	2019			2018			2017
Income tax provision at the statutory rate	$	49.7		$	42.9		$	69.0
State income taxes, net of federal benefit	11.5			9.4			6.8
Disallowed meals and entertainment expenses	1.7			1.6			1.9
Excess stock-based compensation benefit	(0.9		)	(2.2		)	(4.2		)
Work opportunity tax credit	(2.5		)	(3.1		)	(2.0		)
Impact of tax reform	—			(3.0		)	(31.4		)
Other	2.5			0.6			(0.9		)
	$	62.0		$	46.2		$	39.2

As of December 31, 2019, the Company had no domestic net operating losses and had $0.8 million of foreign net operating losses, which have no expiration date. The Company has recorded a valuation allowance of approximately $0.8 million and $1.0 million at December 31, 2019 and 2018, respectively, related to net operating loss carryforwards.

At December 31, 2019, the Company had undistributed earnings of foreign subsidiaries of approximately $16.7 million, substantially all of which are permanently reinvested. The Company will repatriate a portion of these foreign earnings in situations it deems advantageous for business operations, tax or cash management reasons. In doing so, the Company could be subject to state income and foreign taxes which would be insignificant. The determination of the amount of unrecognized deferred income tax liability for any basis differences on the permanently reinvested foreign earnings is not practicable due to the complexities associated with this hypothetical calculation.

The Company had gross deferred tax assets of $58.3 million and $32.5 million and gross deferred tax liabilities of $156.2 million and $111.3 million at December 31, 2019 and 2018, respectively. Management has determined the gross deferred tax assets are realizable, with the exception of foreign net operating losses discussed above.

At December 31, 2019, 2018 and 2017, there were $1.3 million, $0.4 million and $0.4 million of unrecognized tax benefits, respectively, that if recognized would affect the annual effective tax rate. The gross unrecognized tax benefits are included in other long-term liabilities. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. The amount of interest and penalties recognized in the financial statements is not significant. The Company believes that there will be no significant decrease in unrecognized tax benefits by the end of 2020. The following is a reconciliation of the total amounts of unrecognized tax benefits (in millions):



	Year Ended December 31,
	2019		2018			2017
Unrecognized tax benefit beginning of year	$	0.4	$	0.4		$	1.3
Gross increases - tax positions in current year	—		—			0.1
Gross increases - tax positions in prior year	0.9		0.1			—
Gross decreases - tax positions in prior year	—		—			(0.3		)
Lapse of the statute of limitations	—		(0.1		)	(0.7		)
Unrecognized tax benefit end of year	$	1.3	$	0.4		$	0.4

The Company is subject to taxation in the United States and various states and foreign jurisdictions. The IRS has commenced an examination of the Company's U.S. income tax return for the 2017 tax year. The Company remains subject to U.S. federal income tax examinations for 2016 and subsequent years. For major U.S. states, with few exceptions and generally for the foreign tax jurisdictions, the Company remains subject to examination for 2015 and subsequent years.

Earnings per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Earnings per share

Earnings per Share

Basic earnings per share are computed using the weighted average number of shares outstanding and diluted earnings per share are computed using the weighted average number of shares and dilutive share equivalents (consisting of non-qualified stock options, restricted stock units and employee stock purchase plan contributions) outstanding during the periods using the treasury stock method.

The following is a reconciliation of the shares used to compute basic and diluted earnings per share (in millions):



	Year Ended December 31,
	2019	2018	2017
Weighted average number of common shares outstanding used to compute basic earnings per share	52.8	52.3	52.5
Dilutive effect of stock-based awards	0.6	0.8	0.7
Number of shares used to compute diluted earnings per share	53.4	53.1	53.2

There were no significant share equivalents outstanding as of December 31, 2019, 2018 and 2017 that were anti-dilutive when applying the treasury stock method.

Business Segments

12 Months Ended

Dec. 31, 2019

Segment Reporting [Abstract]

Segment Reporting Disclosure [Text Block]

Business Segments

ASGN provides IT and professional staffing services in the technology, digital, creative, engineering and life sciences fields across commercial and government sectors. ASGN operates through its Apex, Oxford and ECS segments. The Apex Segment provides technology, digital, creative, scientific, engineering staffing and consulting services to Fortune 1000 and mid-market clients across the United States and Canada. The businesses in this segment include Apex Systems and Creative Circle. The Oxford Segment provides hard-to-find technology, digital, engineering and life sciences staffing and consulting services, in select skill and geographic markets in the United States and Europe. The businesses in this segment include Oxford Global Resources and CyberCoders. The ECS Segment delivers advanced solutions in cloud, cybersecurity, artificial intelligence, machine learning, application and IT modernization, science and engineering.

The Company evaluates the performance of each segment primarily based on revenues, gross profit and operating income. The information in the following tables is derived directly from the segments’ internal financial reporting used for corporate management purposes.

The following tables present revenues, gross profit, operating income and amortization by reportable segment (in millions):



	Year ended December 31,
	2019			2018			2017
Apex:
Revenues	$	2,520.0		$	2,300.3		$	2,037.2
Gross profit	746.0			687.9			606.3
Operating income	287.7			262.4			222.0
Amortization	20.3			26.2			29.4
Oxford:
Revenues	$	605.7		$	606.5		$	588.8
Gross profit	242.9			248.9			243.8
Operating income	48.4			54.1			53.8
Amortization	3.7			4.2			4.0
ECS:
Revenues	$	798.2		$	493.0		$	—
Gross profit	141.1			86.9			—
Operating income	42.2			15.5			—
Amortization	27.1			28.1			—
Corporate:
Operating loss⁽¹⁾	$	(69.7	)	$	(71.8	)	$	(51.1	)
Consolidated:
Revenues	$	3,923.9		$	3,399.8		$	2,626.0
Gross profit	1,130.0			1,023.7			850.1
Operating income	308.6			260.2			224.7
Amortization	51.1			58.5			33.4

_________

⁽¹⁾ Corporate expenses primarily consist of consolidated stock-based compensation expense, compensation for corporate employees, acquisition, integration and strategic planning expenses, public company expenses and depreciation expense for corporate assets.

The following table presents revenues by type (in millions):



	Year Ended December 31,
	2019		2018		2017
Apex:
Assignment	$	2,463.6	$	2,244.5	$	1,993.3
Permanent placement	56.4		55.8		43.9
	$	2,520.0	$	2,300.3	$	2,037.2
Oxford:
Assignment	$	522.7	$	516.0	$	503.1
Permanent placement	83.0		90.5		85.7
	$	605.7	$	606.5	$	588.8
ECS:
Firm-fixed-price	$	214.0	$	133.1	$	—
Time and Materials	267.8		143.4		—
Cost Reimbursable	316.4		216.5		—
	$	798.2	$	493.0	$	—
Consolidated	$	3,923.9	$	3,399.8	$	2,626.0

The following table presents the ECS segment revenues by customer type (in millions):



	Year Ended December 31,
	2019		2018
Department of Defense and Intelligence Agencies	$	453.9	$	311.0
Federal Civilian	293.6		150.9
Commercial and Other	50.7		31.1
	$	798.2	$	493.0

The Company operates internationally, with operations in the United States, Europe and Canada. The following table presents revenues by geographic location (in millions):



	Year Ended December 31,
	2019		2018		2017
Domestic	$	3,749.2	$	3,241.8	$	2,494.5
Foreign	174.7		158.0		131.5
	$	3,923.9	$	3,399.8	$	2,626.0

The following table presents total assets by reportable segment (in millions):



	December 31,
	2019		2018
Apex	$	1,504.5	$	1,368.5
Oxford	433.8		421.2
ECS	935.2		865.6
Corporate⁽¹⁾	67.9		32.5
	$	2,941.4	$	2,687.8

_________

⁽¹⁾ Corporate segment assets include workers’ compensation receivable and rabbi trust assets related to deferred compensation for employees of other segments. These programs are managed by the Company's corporate office and the related balances are recorded at corporate. These assets have offsetting liabilities for workers' compensation loss reserves and the Company's deferred compensation plan.

The following table presents long-lived assets by geographic location (in millions):



	December 31,
	2019		2018
Domestic	$	71.4	$	76.7
Foreign	2.3		2.4
	$	73.7	$	79.1

Fair Value Measurements	12 Months Ended
Fair Value Measurements	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The carrying amounts of cash and cash equivalents, accounts receivable, accounts payable and accrued payroll and contractor professional pay approximate their fair value based on their short-term nature. Long-term debt recorded in the Company’s consolidated balance sheet at December 31, 2019 was $1.0 billion, excluding the $8.5 million of unamortized deferred loan costs (see Note 7 – Long-Term Debt). The fair value of long-term debt was $1.1 billion as of December 31, 2019 and was determined using quoted prices in active markets for identical liabilities (Level 1 inputs). The Company had investments, primarily mutual funds, of $11.8 million and $6.2 million at December 31, 2019 and 2018, respectively, held in a rabbi trust restricted to fund the Company's deferred compensation plan and are measured at fair value using the net asset value ("NAV") per share. These assets were primarily included in other non-current assets. Certain assets and liabilities, such as goodwill and trademarks, are not measured at fair value on an ongoing basis but are subject to fair value adjustments in certain circumstances (e.g., when there is evidence of impairment). There were no other fair value adjustments for non-financial assets or liabilities in the years ended December 31, 2019, 2018 and 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Unaudited Quarterly Financial Results

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Results

The following tables present unaudited quarterly financial information (in millions, except per share amounts). In the opinion of the Company’s management, the quarterly information contains all adjustments, consisting only of normal recurring accruals, necessary for a fair presentation thereof. The operating results for any quarter are not necessarily indicative of the results for any future periods.



	Quarter Ended									Year Ended Dec. 31,
2019	Mar. 31,		June 30,		Sept. 30,			Dec. 31,		Year Ended Dec. 31,

Revenues	$	923.7	$	972.3	$	1,002.7		$	1,025.2	$	3,923.9
Gross profit	263.9		285.2		291.4			289.5		1,130.0
Income from continuing operations	34.9		43.1		57.5			39.3		174.8
Loss from discontinued operations, net of income taxes	—		—		(0.1		)	—		(0.1		)
Net income	$	34.9	$	43.1	$	57.4		$	39.3	$	174.7

Per share income from continuing operations and net income:
Basic	$	0.66	$	0.82	$	1.09		$	0.75	$	3.31

Diluted earnings per common share:
Diluted	$	0.66	$	0.81	$	1.08		$	0.74	$	3.28



	Quarter Ended											Year Ended Dec. 31,
2018	Mar. 31,			June 30,⁽¹⁾			Sept. 30,		Dec. 31,			Year Ended Dec. 31,

Revenues	$	685.2		$	878.5		$	906.4	$	929.7		$	3,399.8
Gross profit	217.7			263.9			270.1		272.0			1,023.7
Income from continuing operations	29.2			33.7			49.1		46.0			158.0
Income (loss) from discontinued operations, net of income taxes	(0.1		)	(0.1		)	—		(0.1		)	(0.3		)
Net income	$	29.1		$	33.6		$	49.1	$	45.9		$	157.7

Per share income from continuing operations and net income:
Basic	$	0.56		$	0.64		$	0.94	$	0.88		$	3.02

Diluted earnings per common share:
Diluted	$	0.55		$	0.63		$	0.93	$	0.86		$	2.98

______

(1)

ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

Subsequent Event (Notes)	12 Months Ended
Subsequent Event (Notes)	Dec. 31, 2019
Subsequent Event [Line Items]
Subsequent Events [Text Block]	Subsequent Event On January 24, 2020, the Company acquired all of the outstanding equity interests of the federal division of Blackstone Technology Group (“Blackstone Federal”), for $85.0 million in cash. Blackstone Federal is headquartered in Arlington, Virginia. The acquisition expands the Company's capabilities in agile application development, cloud modernization and systems architecture, cybersecurity, user experience design and branding services to government clients. The results of operations of Blackstone Federal, which will be part of the ECS segment, will be included in the consolidated results of the Company the date of its acquisition.

Schedule II - Valuation and Qualifying Accounts

12 Months Ended

Dec. 31, 2019

SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]

SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block]



			Additions
Description	Balance at beginning of year		Charged to costs and expenses	Charged to other accounts⁽²⁾	Deductions⁽³⁾		Balance at end of year
Year ended December 31, 2019
Allowance for doubtful accounts	$	4.8	3.7	—	(3.4	)	$	5.1
Workers’ compensation loss reserves	$	17.4	3.3	—	(4.5	)	$	16.2

Year ended December 31, 2018
Allowance for doubtful accounts and billing adjustments⁽¹⁾	$	8.5	3.3	—	(7.0	)	$	4.8
Workers’ compensation loss reserves	$	14.8	3.6	—	(1.0	)	$	17.4

Year ended December 31, 2017
Allowance for doubtful accounts and billing adjustments	$	8.1	5.3	6.8	(10.3	)	$	9.9
Workers’ compensation loss reserves	$	15.8	2.9	—	(3.9	)	$	14.8

______

(1) Upon adoption of ASC 606, permanent placement fallouts of $1.5 million were reclassified from allowance from doubtful accounts to other current liabilities, effective January 1, 2018.

(2) Charges to other accounts include provision for permanent placement fallouts that have been deducted from net revenues and billing adjustments that have been deducted from net revenues in the accompanying consolidated statements of operations and comprehensive income.

(3) Deductions from allowance for doubtful accounts include write-offs of uncollectible accounts receivable for the years ended December 31, 2019 and 2018. For the year ended December 31, 2017 deductions from allowance for doubtful accounts include write-offs of uncollectible accounts receivable, permanent placements fallouts that have been charged against the allowance for doubtful accounts and billing adjustments. Deductions from workers’ compensation loss reserves include payments of claims and changes related to anticipated insurance and indemnification recoveries for all periods presented.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Principles of Consolidation	Principles of Consolidation. The consolidated financial statements include the accounts of ASGN Incorporated and its wholly-owned subsidiaries (the “Company”). All intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates. The preparation of financial statements in conformity with generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Revenue Recognition	Revenue Recognition. Revenues are recognized as control of the promised service is transferred to customers, in an amount that reflects the consideration expected in exchange for the services. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods, see Note 11 – Revenues.
Cost of Services	Costs of Services. Costs of services include direct costs of contract assignments consisting primarily of payroll, payroll taxes and benefit costs for the Company’s contract professionals. Costs of services also include other direct costs and reimbursable out-of-pocket expenses.
Income Taxes	Income Taxes. Income taxes are accounted for using the liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that a portion of the deferred tax asset will not be realized. The Company reviews its uncertain tax positions regularly. An uncertain tax position represents the Company’s expected treatment of a tax position taken in a filed return, or planned to be taken in a future tax return or claim that has not been reflected in measuring income tax expense for financial reporting purposes. The Company recognizes the tax benefit from an uncertain tax position when it is more-likely-than-not that the position will be sustained upon examination on the basis of the technical merits or the statute of limitations for the relevant taxing authority to examine and challenge the tax position has expired.
Foreign Currency Translation	Foreign Currency Translation. The functional currency of the Company’s foreign operations is their local currency, and as such, their assets and liabilities are translated into U.S. dollars at the rate of exchange in effect on the balance sheet date. Revenues and expenses are translated at the average rates of exchange prevailing during each monthly period. The related translation adjustments are recorded as cumulative foreign currency translation adjustments in accumulated other comprehensive income (loss) as a separate component of stockholders’ equity. Gains and losses resulting from foreign currency transactions, which are not material, are included in selling, general and administrative (“SG&A”) expenses in the consolidated statements of operations and comprehensive income.
Cash and Cash Equivalents	Cash and Cash Equivalents. The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents.
Accounts Receivable Allowances	Accounts Receivable Allowances. The Company estimates an allowance for expected credit losses (the inability of customers to make required payments). These estimates are based on a combination of past experience and current trends. In estimating the allowance for expected credit losses, consideration is given to the current aging of receivables and a specific review for potential bad debts. The resulting bad debt expense is included in SG&A expenses in the consolidated statements of operations and comprehensive income. Receivables are written-off when deemed uncollectible.
Leases	Leases. Effective January 1, 2019, operating leases are included in operating lease right of use assets and operating lease liabilities on the consolidated balance sheets, see Note 2 – Accounting Standards Update. Effective January 1, 2019, the Company adopted Accounting Standards Update ("ASU") 2016-02 Leases (Accounting Standards Codification Topic "ASC" 842), which requires lessees to recognize most operating leases on the balance sheet as a right of use ("ROU") asset and lease liability. The Company adopted this standard using the optional transition method measuring and recognizing the ROU asset and lease liability from operating leases on the condensed consolidated balance sheet without comparative period information or disclosures. The adoption of the standard did not have an effect on the Company’s results of operations, stockholders' equity or cash flows. The Company elected the package of practical expedients which specifies entities do not need to reassess expired or existing contracts as of the adoption date for the following items: (i) determination of whether a contract is or contains a lease, (ii) revising classification of leases and (iii) assessment of initial direct costs. For existing or expired contracts as of the adoption date, the determinations made for these items under the previous accounting standard (ASC 840) were retained at transition, as allowed by this package of practical expedients. The Company has operating leases for corporate offices, branch offices and data centers. At the transition date, the operating lease ROU asset and operating lease liability were $93.9 million and $99.4 million, respectively. The difference between the operating lease ROU asset and operating lease liability is due to deferred rent and prepaid rent balances that were reclassified as a component of the ROU asset at the transition date. The Company's leases have remaining lease terms of one month to eight years. At the inception of a contract, the Company determines if the contract contains a lease. A contract contains a lease if it conveys the right to control the use of an identified asset for a period of time in exchange for consideration. Operating lease ROU assets and operating lease liabilities are recognized at the lease commencement date, based on the present value of the future minimum lease payments. The Company’s leases do not provide an implicit rate of return. Therefore, the Company uses its incremental borrowing rate (“IBR”) in determining the present value of lease payments. In determining the IBR, the Company considers its credit rating and the current market interest rates. The IBR approximates the interest rate the Company would pay on collateralized debt with similar terms and payments as the lease agreements and in a similar economic environment where the leased assets are located. Leases with an initial term of 12 months or less ("short-term leases") are not recorded on the balance sheet. The Company does not have finance leases. Lease expense is recognized on a straight-line basis over the lease term and is primarily included in selling, general and administrative expenses. Some lease agreements offer renewal options which are assessed against relevant economic factors to determine whether it is reasonably certain that these renewal options will be exercised. As a result of this assessment, for most leases, renewal options were excluded from the minimum lease payments when calculating the operating lease ROU assets and operating lease liabilities, as the Company does not consider the exercise of such options to be reasonably certain. The Company has lease agreements with lease and non-lease components, which are accounted for as a single lease component for all underlying asset classes. Some leases require variable payments for common area maintenance, property taxes, parking, insurance and other variable costs. The variable portion of lease payments is not included in operating lease ROU assets or operating lease liabilities. Variable lease costs are expensed when incurred.
Property and Equipment	Property and Equipment. Property and equipment are stated at cost. Depreciation and amortization are provided using the straight-line method over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized over the shorter of the life of the related asset or the remaining term of the lease. Costs associated with customized internal-use software systems that have reached the application development stage and meet recoverability tests are capitalized and include external direct costs utilized in developing or obtaining the applications and payroll and payroll-related expenses for employees who are directly associated with the application development.
Business Combinations	Business Combinations and Related Acquired Intangible Assets and Goodwill. The Company records all tangible and intangible assets acquired and liabilities assumed in a business combination at fair value as of the acquisition date in accordance with Accounting Standards Codification ("ASC") 805 Business Combinations. Acquisition date fair value represents the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants as measured on the acquisition date. The valuations are based on information that existed as of the acquisition date. During the measurement period, which shall not exceed one year from the acquisition date, provisional amounts may be adjusted to reflect new information the Company has subsequently obtained regarding facts and circumstances that existed as of the acquisition date. Such fair value assessments require judgments and estimates, which may cause final amounts to differ materially from original estimates. The excess amount of the aggregated purchase consideration paid over the fair value of the net of assets acquired and liabilities assumed is recorded as goodwill. Goodwill is evaluated for impairment annually or more frequently if an event occurs or circumstances change, such as material deterioration in performance that would indicate an impairment may exist. The Company performs the annual impairment assessment as of October 31^stfor each of its four reporting units. When evaluating goodwill for impairment, the Company may first perform a qualitative assessment to determine whether it is more likely than not that a reporting unit is impaired. If the Company decides not to perform a qualitative assessment, or if it determines that it is more likely than not that the carrying amount of a reporting unit exceeds its fair value, then the Company performs a quantitative assessment and calculates the estimated fair value of the reporting unit. If the carrying amount of the reporting unit exceeds the estimated fair value, an impairment charge would be recorded to reduce the carrying amount to its estimated fair value. The decision to perform a qualitative impairment assessment in a given year is influenced by a number of factors, including the significance of the excess of the reporting units’ estimated fair value over carrying amount at the last quantitative assessment date, the amount of time in between quantitative fair value assessments and the date of the acquisitions. A qualitative assessment is performed for trademarks to determine if there are any indicators that the carrying amount might not be recovered. A quantitative analysis may be performed in order to test the trademarks for impairment. If a quantitative analysis is necessary, an income approach, specifically a relief from royalty method, is used to estimate the fair value of the trademarks. Principal factors used in the relief from royalty method that require judgment are projected net sales, discount rates, royalty rates and terminal growth assumptions. The estimated fair value of each trademark is compared to its carrying amount to determine if impairment exists. If the carrying amount of a trademark exceeds the estimated fair value, an impairment charge would be recorded to reduce the carrying amount of the trademark. Finite-lived intangible assets are amortized over their useful lives and are tested for recoverability whenever events or changes in circumstances indicate that the carrying amount may not be recoverable. Customer and contractual relationships, backlog and favorable contracts are amortized based on the annual cash flows observed in the valuation of the asset, which generally accelerates the amortization into the earlier years reflective of the economic life of the asset. Contractor relationships, non-compete agreements and in-use software are amortized using the straight-line method.
Impairment or Disposal of Long-Lived Assets	Impairment or Disposal of Long-Lived Assets. The Company evaluates long-lived assets, other than goodwill and identifiable intangible assets with indefinite lives, for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. An impairment loss is recognized when the sum of the undiscounted future cash flows is less than the carrying amount of the asset, in which case a write-down is recorded to reduce the related asset to its estimated fair value. There were no significant impairments of long-lived assets in 2019, 2018 and 2017.
workers compensation loss reserve	Workers’ Compensation Loss Reserves. The Company carries retention policies for its workers’ compensation liability exposures. Under these policies, the Company pays a base premium plus actual losses incurred, not to exceed certain stop-loss limits. The Company is insured for losses above these limits. The Company estimates its workers' compensation loss reserves based on a third- party actuarial study based on claims filed and claims incurred but not reported. The Company accounts for claims incurred but not yet reported based on estimates derived from historical claims experience and current trends of industry data. Changes in estimates and differences in estimates and actual payments for claims are recognized in the period that the estimates changed or the payments were made.
Contingencies	Contingencies. The Company records an estimated loss from a loss contingency when information available prior to issuance of its financial statements indicates it is probable that an asset has been impaired or a liability has been incurred at the date of the financial statements and the amount of the loss can be reasonably estimated. Accounting for contingencies, such as legal settlements and workers’ compensation matters, requires the Company to use judgment.
Stock-based Compensation	Stock-Based Compensation. The Company records compensation expense for restricted stock awards and restricted stock units based on the fair market value of the awards on the date of grant. The fair value of stock-based compensation awards less the estimated forfeitures is amortized over the vesting period of the award. Compensation expense for performance-based awards is measured based on the amount of shares ultimately expected to vest, estimated at each reporting date based on management’s expectations regarding the relevant performance criteria. The Company accounts for employee stock purchase plan shares based on an estimated fair market value using a Black-Scholes valuation model. This methodology requires the use of subjective assumptions such as expected stock price volatility.
Concentration of Credit Risk	Concentration of Credit Risk. Financial instruments that potentially subject the Company to credit risks consist primarily of cash and cash equivalents and trade receivables. The Company places its cash and cash equivalents with high quality financial institutions. Concentration of credit risk with respect to accounts receivable for the Apex and Oxford segments is limited because of the large number of clients and their dispersion across different industries and geographies, thus spreading the trade credit risk. The Company performs ongoing credit evaluations to identify risks and maintains an allowance to address these risks. Accounts receivables from the ECS segment are primarily from the U.S. government and are considered to have low credit risk.

Leases (Policies)	12 Months Ended
Leases (Policies)	Dec. 31, 2019
Leases [Abstract]
Leases	Leases. Effective January 1, 2019, operating leases are included in operating lease right of use assets and operating lease liabilities on the consolidated balance sheets, see Note 2 – Accounting Standards Update. Effective January 1, 2019, the Company adopted Accounting Standards Update ("ASU") 2016-02 Leases (Accounting Standards Codification Topic "ASC" 842), which requires lessees to recognize most operating leases on the balance sheet as a right of use ("ROU") asset and lease liability. The Company adopted this standard using the optional transition method measuring and recognizing the ROU asset and lease liability from operating leases on the condensed consolidated balance sheet without comparative period information or disclosures. The adoption of the standard did not have an effect on the Company’s results of operations, stockholders' equity or cash flows. The Company elected the package of practical expedients which specifies entities do not need to reassess expired or existing contracts as of the adoption date for the following items: (i) determination of whether a contract is or contains a lease, (ii) revising classification of leases and (iii) assessment of initial direct costs. For existing or expired contracts as of the adoption date, the determinations made for these items under the previous accounting standard (ASC 840) were retained at transition, as allowed by this package of practical expedients. The Company has operating leases for corporate offices, branch offices and data centers. At the transition date, the operating lease ROU asset and operating lease liability were $93.9 million and $99.4 million, respectively. The difference between the operating lease ROU asset and operating lease liability is due to deferred rent and prepaid rent balances that were reclassified as a component of the ROU asset at the transition date. The Company's leases have remaining lease terms of one month to eight years. At the inception of a contract, the Company determines if the contract contains a lease. A contract contains a lease if it conveys the right to control the use of an identified asset for a period of time in exchange for consideration. Operating lease ROU assets and operating lease liabilities are recognized at the lease commencement date, based on the present value of the future minimum lease payments. The Company’s leases do not provide an implicit rate of return. Therefore, the Company uses its incremental borrowing rate (“IBR”) in determining the present value of lease payments. In determining the IBR, the Company considers its credit rating and the current market interest rates. The IBR approximates the interest rate the Company would pay on collateralized debt with similar terms and payments as the lease agreements and in a similar economic environment where the leased assets are located. Leases with an initial term of 12 months or less ("short-term leases") are not recorded on the balance sheet. The Company does not have finance leases. Lease expense is recognized on a straight-line basis over the lease term and is primarily included in selling, general and administrative expenses. Some lease agreements offer renewal options which are assessed against relevant economic factors to determine whether it is reasonably certain that these renewal options will be exercised. As a result of this assessment, for most leases, renewal options were excluded from the minimum lease payments when calculating the operating lease ROU assets and operating lease liabilities, as the Company does not consider the exercise of such options to be reasonably certain. The Company has lease agreements with lease and non-lease components, which are accounted for as a single lease component for all underlying asset classes. Some leases require variable payments for common area maintenance, property taxes, parking, insurance and other variable costs. The variable portion of lease payments is not included in operating lease ROU assets or operating lease liabilities. Variable lease costs are expensed when incurred.

Leases (Tables)

12 Months Ended

Dec. 31, 2019

Dec. 31, 2018

Leases [Abstract]

Contractual Obligation, Fiscal Year Maturity [Table Text Block]

The following is a summary of the Company's purchase obligations as of December 31, 2019, excluding lease liabilities (see Note 3 – Leases) and other current liabilities that are included in the consolidated balance sheet (in millions):



	Purchase Obligations
2020	$	13.4
2021	11.6
2022	11.3
2023	4.4
2024	—
Thereafter	—
Total	$	40.7

As previously disclosed in the Company's 2018 Annual Report on Form 10-K under the previous lease accounting standard, minimum rental commitments under non-cancelable operating leases in effect at December 31, 2018 were (in millions):



	Operating Leases		Related Party Leases		Total
2019	$	29.6	$	1.3	$	30.9
2020	24.1		1.3		25.4
2021	19.8		1.4		21.2
2022	14.2		1.4		15.6
2023	8.9		1.4		10.3
Thereafter	6.3		1.1		7.4
Total	$	102.9	$	7.9	$	110.8

Lease, Cost [Table Text Block]

Components of lease expense for the year ended December 31, 2019 were as follows (in millions):



Operating lease expense	$	32.1
Short-term lease expense	2.1
Variable lease expense	5.8
Total lease expense	$	40.0

Cash Flow, Supplemental Disclosures [Text Block]

Supplemental cash flow information related to leases for December 31, 2019 (in millions):



Cash paid for operating lease liabilities	$	32.1
Operating lease ROU assets obtained in exchange for new operating lease liabilities	$	30.0
Weighted-average remaining lease term of operating leases	4.2 years
Weighted-average discount rate of operating leases	4.26		%

Lessee, Operating Lease, Liability, Maturity [Table Text Block]

Maturities of operating lease liabilities at December 31, 2019 (in millions):



2020	$	29.5
2021	28.1
2022	22.1
2023	16.1
2024	9.3
Thereafter	5.6
Total future minimum lease payments	110.7
Less imputed interest	9.2
Total operating lease liabilities	$	101.5

Acquisitions (Tables)

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block]



Cash	$	12.4
Accounts receivable	97.2
Prepaid expenses and other current assets	8.6
Property and equipment	29.0
Identifiable intangible assets	195.0
Goodwill	528.2
Other non-current assets	1.2
Total assets acquired	871.6

Current liabilities	94.7
Long-term liabilities	4.3
Total liabilities assumed	99.0

Total purchase price	$	772.6

Finite-Lived and Indefinite-Lived Intangible Assets Acquired as Part of Business Combination [Table Text Block]

The following table summarizes the acquired identifiable intangible assets of ECS (in millions):



	Useful life
Contractual customer relationships	12.75 years	$	144.6
Backlog	2.75 years	23.1
Non-compete agreements	4 to 7 years	10.3
Favorable contracts	5 years	0.5
Trademarks	indefinite	16.5
Total identifiable intangible assets acquired		$	195.0

Business Acquisition, Pro Forma Information [Table Text Block]

The summary below (in millions, except for per share data) presents pro forma unaudited consolidated results of operations for the years ended December 31, 2018 and 2017 as if the acquisition of ECS by the Company and the acquisition of a business by ECS in April 2017, both occurred on January 1, 2017. The pro forma unaudited consolidated results give effect to, among other things: (i) amortization of intangible assets, (ii) stock-based compensation expense and the related dilution for restricted stock units granted to ECS employees, (iii) interest expense on acquisition-related debt and (iv) the exclusion of nonrecurring expenses incurred by ECS prior to its acquisition by the Company for ECS’ acquisition-related activities and costs incurred in the sale of ECS to the Company. The pro forma results do not include pre-acquisition results of DHA or Intersys due to their size. The pro forma unaudited consolidated results are not necessarily indicative of the operating results that would have occurred if the acquisition had been consummated as of the date indicated, nor are they necessarily indicative of future operating results.



	2018		2017
Revenues	$	3,548.9	$	3,213.5
Income from continuing operations	$	169.6	$	134.8
Net income	$	169.3	$	134.6

Earnings per share:
Basic	$	3.24	$	2.57
Diluted	$	3.19	$	2.53

Number of shares and share equivalents used to calculate earnings per share:
Basic	52.4		52.5
Diluted	53.2		53.2

Goodwill and Identifiable Assets (Tables)

12 Months Ended

Dec. 31, 2019

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Goodwill

The changes in the carrying amount of goodwill for the years ended December 31, 2019 and 2018 are as follows (in millions):



	Apex Segment		Oxford Segment			ECS Segment		Total
Balance as of December 31, 2017	$	662.1	$	232.0		$	—	$	894.1
ECS acquisition	—		—			528.2		528.2
Translation adjustment	—		(1.2		)	—		(1.2		)
Balance as of December 31, 2018	662.1		230.8			528.2		1,421.1
DHA acquisition	—		—			24.7		24.7
Intersys acquisition	41.4		—			—		41.4
Translation adjustment	—		(0.3		)	—		(0.3		)
Balance as of December 31, 2019	$	703.5	$	230.5		$	552.9	$	1,486.9

Schedule of Acquired Intangible Assets

Acquired intangible assets consisted of the following (in millions):



		December 31, 2019						December 31, 2018
	Estimated Useful Life	Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount
Subject to amortization:
Customer and contractual relationships	2 - 12.75 years	$	384.9	$	179.9	$	205.0	$	346.9	$	145.4	$	201.5
Contractor relationships	2 - 5 years	71.1		70.6		0.5		71.1		67.1		4.0
Backlog	1 - 2.75 years	25.0		23.9		1.1		23.1		17.7		5.4
Non-compete agreements	2 - 7 years	24.8		13.8		11.0		22.1		9.9		12.2
In-use software	6 years	18.9		18.9		—		18.9		16.0		2.9
Favorable contracts	5 years	—		—		—		1.4		0.9		0.5
		524.7		307.1		217.6		483.5		257.0		226.5
Not subject to amortization:
Trademarks⁽¹⁾		258.9		—		258.9		262.2		—		262.2
Total		$	783.6	$	307.1	$	476.5	$	745.7	$	257.0	$	488.7

Schedule of Estimated Future Amortization Expense

Estimated amortization for each of the next five years and thereafter follows (in millions):



2020	$	44.7
2021	37.8
2022	29.4
2023	25.4
2024	19.1
Thereafter	61.2
	$	217.6

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment at December 31, 2019 and 2018 consisted of the following (in millions):



	2019			2018
Computer hardware and software	$	180.2		$	159.4
Furniture, fixtures and equipment	26.8			22.2
Leasehold improvements	24.7			19.2
Work-in-progress	7.0			3.7
	238.7			204.5
Less -- accumulated depreciation	(165.0		)	(125.4		)
	$	73.7		$	79.1

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Schedule of Long-term Debt

At December 31, 2019 and 2018, long-term debt consisted of the following (in millions):



	2019			2018
Senior Credit Facility:
$250 million revolving credit facility, due November 22, 2024	$	—		$	—
Term B loan facility, due June 5, 2022	—			337.0
Term B loan facility, due April 2, 2025	490.8			787.0
Senior Notes, due May 15, 2028	550.0			—
	1,040.8			1,124.0
Unamortized deferred loan costs	(8.5		)	(23.6		)
	$	1,032.3		$	1,100.4

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

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+ Details

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2019

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Schedule of Contractual Cash Obligation Payments

The following is a summary of the Company's purchase obligations as of December 31, 2019, excluding lease liabilities (see Note 3 – Leases) and other current liabilities that are included in the consolidated balance sheet (in millions):



	Purchase Obligations
2020	$	13.4
2021	11.6
2022	11.3
2023	4.4
2024	—
Thereafter	—
Total	$	40.7

As previously disclosed in the Company's 2018 Annual Report on Form 10-K under the previous lease accounting standard, minimum rental commitments under non-cancelable operating leases in effect at December 31, 2018 were (in millions):



	Operating Leases		Related Party Leases		Total
2019	$	29.6	$	1.3	$	30.9
2020	24.1		1.3		25.4
2021	19.8		1.4		21.2
2022	14.2		1.4		15.6
2023	8.9		1.4		10.3
Thereafter	6.3		1.1		7.4
Total	$	102.9	$	7.9	$	110.8

Stock-based Compensation and Other Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Share-based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]

A summary of the status of the Company’s unvested RSUs as of December 31, 2019 and changes during the year then ended are presented below (number of units in millions, except fair value per unit):



	Service Conditions RSUs		Performance and Service Conditions RSUs		Total RSUs		Weighted Average Grant-Date Fair Value Per Unit
Unvested RSUs outstanding at December 31, 2018	0.4		0.9		1.3			$	60.87
Granted	0.2		0.4		0.6			$	62.26
Vested	(0.1	)	(0.5	)	(0.6	)		$	56.58
Forfeited	—		(0.1	)	(0.1	)		$	67.58
Unvested RSUs outstanding at December 31, 2019	0.5		0.7		1.2			$	63.21
Unvested and expected to vest RSUs outstanding at December 31, 2019	0.4		0.7		1.1			$	62.96

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]

The table below presents the average Black-Scholes valuation per share of shares purchased and the compensation expense under the ESPP (in millions, except per share amounts):



Year Ended December 31,	Average Black-Scholes Valuation per Share	Shares	Expense
2019	$17.11	0.2	$	4.1
2018	$15.09	0.2	$	2.7
2017	$9.71	0.2	$	2.1

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Components of Income Taxes

The provision for income taxes consists of the following (in millions):



	Year Ended December 31,
	2019			2018			2017
Current:
Federal	$	29.9		$	20.6		$	33.6
State	8.6			10.6			7.7
Foreign	4.6			3.9			2.8
	43.1			35.1			44.1
Deferred:
Federal and State	19.9			11.5			(4.7		)
Foreign	(1.0		)	(0.4		)	(0.2		)
	18.9			11.1			(4.9		)
	$	62.0		$	46.2		$	39.2

Income (Loss) before Income Tax Provision

Income from continuing operations before income taxes consists of the following (in millions):



	Year Ended December 31,
	2019		2018		2017
United States	$	218.7	$	190.7	$	184.2
Foreign	18.1		13.5		12.9
	$	236.8	$	204.2	$	197.1

Schedule of Deferred Tax Assets and Liabilities

The components of deferred tax assets (liabilities) are as follows (in millions):



	December 31,
	2019			2018
Intangibles	$	(112.7	)	$	(90.8	)
Depreciation expense	(13.3		)	(15.0		)
Operating lease right of use assets	(24.8		)	—
Operating lease liabilities	25.7			—
Allowance for doubtful accounts	1.8			1.6
Employee-related accruals	12.0			9.6
Stock-based compensation	9.2			9.5
Other	3.4			5.3
Net operating loss carryforwards–foreign	0.8			1.0
Valuation allowance	(0.8		)	(1.0		)
	$	(98.7	)	$	(79.8	)

Schedule of Effective Income Tax Rate Reconciliation

The reconciliation between the amount computed by applying the U.S. federal statutory tax rate of 21 percent in 2019 and 2018 and 35 percent in 2017 to income before income taxes, for each respective year and the income tax provision is as follows (in millions):



	Year Ended December 31,
	2019			2018			2017
Income tax provision at the statutory rate	$	49.7		$	42.9		$	69.0
State income taxes, net of federal benefit	11.5			9.4			6.8
Disallowed meals and entertainment expenses	1.7			1.6			1.9
Excess stock-based compensation benefit	(0.9		)	(2.2		)	(4.2		)
Work opportunity tax credit	(2.5		)	(3.1		)	(2.0		)
Impact of tax reform	—			(3.0		)	(31.4		)
Other	2.5			0.6			(0.9		)
	$	62.0		$	46.2		$	39.2

Schedule of Unrecognized Tax Benefits Roll Forward



	Year Ended December 31,
	2019		2018			2017
Unrecognized tax benefit beginning of year	$	0.4	$	0.4		$	1.3
Gross increases - tax positions in current year	—		—			0.1
Gross increases - tax positions in prior year	0.9		0.1			—
Gross decreases - tax positions in prior year	—		—			(0.3		)
Lapse of the statute of limitations	—		(0.1		)	(0.7		)
Unrecognized tax benefit end of year	$	1.3	$	0.4		$	0.4

Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Reconciliation of Shares Used to Compute Basic and Diluted Earnings per Share

The following is a reconciliation of the shares used to compute basic and diluted earnings per share (in millions):



	Year Ended December 31,
	2019	2018	2017
Weighted average number of common shares outstanding used to compute basic earnings per share	52.8	52.3	52.5
Dilutive effect of stock-based awards	0.6	0.8	0.7
Number of shares used to compute diluted earnings per share	53.4	53.1	53.2

Business Segments (Tables)

12 Months Ended

Dec. 31, 2019

Revenue from External Customer [Line Items]

Schedule of Segment Reporting Information, by Segment [Table Text Block]

The following tables present revenues, gross profit, operating income and amortization by reportable segment (in millions):



	Year ended December 31,
	2019			2018			2017
Apex:
Revenues	$	2,520.0		$	2,300.3		$	2,037.2
Gross profit	746.0			687.9			606.3
Operating income	287.7			262.4			222.0
Amortization	20.3			26.2			29.4
Oxford:
Revenues	$	605.7		$	606.5		$	588.8
Gross profit	242.9			248.9			243.8
Operating income	48.4			54.1			53.8
Amortization	3.7			4.2			4.0
ECS:
Revenues	$	798.2		$	493.0		$	—
Gross profit	141.1			86.9			—
Operating income	42.2			15.5			—
Amortization	27.1			28.1			—
Corporate:
Operating loss⁽¹⁾	$	(69.7	)	$	(71.8	)	$	(51.1	)
Consolidated:
Revenues	$	3,923.9		$	3,399.8		$	2,626.0
Gross profit	1,130.0			1,023.7			850.1
Operating income	308.6			260.2			224.7
Amortization	51.1			58.5			33.4

_________

⁽¹⁾ Corporate expenses primarily consist of consolidated stock-based compensation expense, compensation for corporate employees, acquisition, integration and strategic planning expenses, public company expenses and depreciation expense for corporate assets.

Disaggregation of Revenue [Table Text Block]

The following table presents revenues by type (in millions):



	Year Ended December 31,
	2019		2018		2017
Apex:
Assignment	$	2,463.6	$	2,244.5	$	1,993.3
Permanent placement	56.4		55.8		43.9
	$	2,520.0	$	2,300.3	$	2,037.2
Oxford:
Assignment	$	522.7	$	516.0	$	503.1
Permanent placement	83.0		90.5		85.7
	$	605.7	$	606.5	$	588.8
ECS:
Firm-fixed-price	$	214.0	$	133.1	$	—
Time and Materials	267.8		143.4		—
Cost Reimbursable	316.4		216.5		—
	$	798.2	$	493.0	$	—
Consolidated	$	3,923.9	$	3,399.8	$	2,626.0

Revenue from External Customers by Products and Services [Table Text Block]

The following table presents the ECS segment revenues by customer type (in millions):



	Year Ended December 31,
	2019		2018
Department of Defense and Intelligence Agencies	$	453.9	$	311.0
Federal Civilian	293.6		150.9
Commercial and Other	50.7		31.1
	$	798.2	$	493.0

Revenue from External Customers by Geographic Areas [Table Text Block]

The Company operates internationally, with operations in the United States, Europe and Canada. The following table presents revenues by geographic location (in millions):



	Year Ended December 31,
	2019		2018		2017
Domestic	$	3,749.2	$	3,241.8	$	2,494.5
Foreign	174.7		158.0		131.5
	$	3,923.9	$	3,399.8	$	2,626.0

Reconciliation of Assets from Segment to Consolidated [Table Text Block]

The following table presents total assets by reportable segment (in millions):



	December 31,
	2019		2018
Apex	$	1,504.5	$	1,368.5
Oxford	433.8		421.2
ECS	935.2		865.6
Corporate⁽¹⁾	67.9		32.5
	$	2,941.4	$	2,687.8

Long-lived Assets by Geographic Areas [Table Text Block]

The following table presents long-lived assets by geographic location (in millions):



	December 31,
	2019		2018
Domestic	$	71.4	$	76.7
Foreign	2.3		2.4
	$	73.7	$	79.1

Unaudited Quarterly Financial Results (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Financial Information



	Quarter Ended									Year Ended Dec. 31,
2019	Mar. 31,		June 30,		Sept. 30,			Dec. 31,		Year Ended Dec. 31,

Revenues	$	923.7	$	972.3	$	1,002.7		$	1,025.2	$	3,923.9
Gross profit	263.9		285.2		291.4			289.5		1,130.0
Income from continuing operations	34.9		43.1		57.5			39.3		174.8
Loss from discontinued operations, net of income taxes	—		—		(0.1		)	—		(0.1		)
Net income	$	34.9	$	43.1	$	57.4		$	39.3	$	174.7

Per share income from continuing operations and net income:
Basic	$	0.66	$	0.82	$	1.09		$	0.75	$	3.31

Diluted earnings per common share:
Diluted	$	0.66	$	0.81	$	1.08		$	0.74	$	3.28



	Quarter Ended											Year Ended Dec. 31,
2018	Mar. 31,			June 30,⁽¹⁾			Sept. 30,		Dec. 31,			Year Ended Dec. 31,

Revenues	$	685.2		$	878.5		$	906.4	$	929.7		$	3,399.8
Gross profit	217.7			263.9			270.1		272.0			1,023.7
Income from continuing operations	29.2			33.7			49.1		46.0			158.0
Income (loss) from discontinued operations, net of income taxes	(0.1		)	(0.1		)	—		(0.1		)	(0.3		)
Net income	$	29.1		$	33.6		$	49.1	$	45.9		$	157.7

Per share income from continuing operations and net income:
Basic	$	0.56		$	0.64		$	0.94	$	0.88		$	3.02

Diluted earnings per common share:
Diluted	$	0.55		$	0.63		$	0.93	$	0.86		$	2.98

______

(1)

ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

Summary of Significant Accounting Policies Lease Term (Details)	Dec. 31, 2019
Minimum [Member]
Leases
Lessee, Operating Lease, Term of Contract	1 month
Maximum [Member]
Leases
Lessee, Operating Lease, Term of Contract	8 years

Leases (Details) $ in Millions	12 Months Ended
Leases (Details) $ in Millions	Dec. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Other Commitments [Line Items]
Operating lease right of use assets	$ 94.6	$ 0.0		$ 93.9
Operating Lease, Liability	$ 101.5			$ 99.4
Number of Leased Properties Owned by Related Parties \| property	2
Costs and Expenses, Related Party	$ 1.2	1.3
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	5 years 3 months 18 days
Operating Leases, Rent Expense		$ 32.7	$ 26.4
Operating Real Estate Leases
Other Commitments [Line Items]
Lessee, Operating Leases, Leases Not Yet Commenced Value	$ 2.0

Leases Contractual Cash Obligations (Details) $ in Millions	Dec. 31, 2018 USD ($)
Operating Leases [Abstract]
2020	$ 29.6
2021	24.1
2022	19.8
2023	14.2
2024	8.9
Thereafter	6.3
Total	102.9
Related Party Leases
2020	1.3
2021	1.3
2022	1.4
2023	1.4
2024	1.4
Thereafter	1.1
Total	7.9
Contractual Obligations [Abstract]
2020	30.9
2021	25.4
2022	21.2
2024	10.3
2023	15.6
Thereafter	7.4
Total	$ 110.8

Leases Lease Cost (Details) $ in Millions	12 Months Ended
Leases Lease Cost (Details) $ in Millions	Dec. 31, 2019 USD ($)
Lease, Cost [Abstract]
Operating Lease, Expense	$ 32.1
Short-term Lease, Expense	2.1
Variable Lease, Expense	5.8
Total Lease, Expense	$ 40.0

Leases Supplemental Information (Details) $ in Millions	12 Months Ended
Leases Supplemental Information (Details) $ in Millions	Dec. 31, 2019 USD ($) Rate
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Operating Lease, Payments	$ 32.1
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 30.0
Operating Lease, Weighted Average Remaining Lease Term	4 years 2 months 12 days
Operating Lease, Weighted Average Discount Rate, Percent \| Rate	4.26%

Leases Operating Lease Liability Payments Due (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020	$ 29.5
2021	28.1
2022	22.1
2023	16.1
2024	9.3
Thereafter	5.6
Future minimum lease payments	110.7
Imputed interest	(9.2)
Operating Lease, Liability	$ 101.5	$ 99.4

Leases Lease Terms (Details)	Dec. 31, 2019
Minimum [Member]
Leases
Lessee, Operating Lease, Term of Contract	1 month
Maximum [Member]
Leases
Lessee, Operating Lease, Term of Contract	8 years

Acquisitions Acquisition Costs, by Acquisition (Details) - USD ($)
$ in Millions

3 Months Ended

12 Months Ended

Oct. 17, 2019

Jan. 25, 2019

Apr. 02, 2018

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

^[1]

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Business Acquisition [Line Items]

Goodwill

$ 1,486.9

$ 1,421.1

$ 1,486.9

$ 1,421.1

Goodwill, Acquired During Period

528.2

Revenues

$ 1,025.2

$ 1,002.7

$ 972.3

$ 923.7

$ 929.7

$ 906.4

$ 878.5

$ 685.2

3,923.9

3,399.8

$ 2,626.0

Intersys

Business Acquisition [Line Items]

Business Acquisition, Effective Date of Acquisition

Oct. 17, 2019

Business Acquisition, Name of Acquired Entity

Intersys Consulting, LLC

Payments to Acquire Businesses, Gross

$ 67.0

Business Acquisition, Description of Acquired Entity

headquartered in Austin, Texas

Business Combination, Reason for Business Combination

The acquisition expands the Company's capabilities in digital innovation and enterprise solutions

Goodwill, Acquired During Period

$ 41.4

41.4

Business Acquisition, Goodwill, Expected Tax Deductible Amount

38.7

Identifiable intangible assets

$ 23.8

Acquired Finite-lived Intangible Assets, Weighted Average Useful Life

9 years

DHA

Business Acquisition [Line Items]

Business Acquisition, Effective Date of Acquisition

Jan. 25, 2019

Business Acquisition, Name of Acquired Entity

DHA Group, Inc

Payments to Acquire Businesses, Gross

$ 46.0

Business Acquisition, Description of Acquired Entity

headquartered in Washington, D.C.

Business Combination, Reason for Business Combination

DHA was acquired to expand the Company's government services

Goodwill, Acquired During Period

$ 24.7

24.7

Identifiable intangible assets

$ 19.0

Acquired Finite-lived Intangible Assets, Weighted Average Useful Life

9 years

Apex

Business Acquisition [Line Items]

Revenues

2,520.0

2,300.3

2,037.2

Apex | Intersys

Business Acquisition [Line Items]

Goodwill, Acquired During Period

41.4

ECS

Business Acquisition [Line Items]

Business Acquisition, Effective Date of Acquisition

Apr. 02, 2018

Business Acquisition, Name of Acquired Entity

ECS Federal, LLC

Payments to Acquire Businesses, Gross

$ 775.0

Business Acquisition, Transaction Costs

$ 12.0

Business Acquisition, Description of Acquired Entity

headquartered in Fairfax, Virginia

Business Combination, Reason for Business Combination

The ECS acquisition allows the Company to compete in the federal IT and professional services sector

Goodwill

$ 528.2

Revenues

798.2

493.0

0.0

Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual

798.2

493.0

$ 0.0

Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual

$ 14.2

Business Acquisition, Goodwill, Expected Tax Deductible Amount

514.2

Identifiable intangible assets

$ 195.0

Acquired Finite-lived Intangible Assets, Weighted Average Useful Life

11 years

ECS | DHA

Business Acquisition [Line Items]

Goodwill, Acquired During Period

$ 24.7

[1]	ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

Acquisitions Schedule of Purchase Price Allocation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Apr. 02, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,421.1		$ 1,486.9
ECS
Business Acquisition [Line Items]
Revenues	3,548.9	$ 3,213.5
Cash				$ 12.4
Accounts receivable				97.2
Prepaid expenses and other current assets				8.6
Property and equipment				29.0
Identifiable intangible assets				195.0
Goodwill				528.2
Other non-current assets				1.2
Total assets acquired				871.6
Current liabilities				94.7
Long-term liabilities				4.3
Total liabilities assumed				99.0
Total purchase price				$ 772.6
Income from continuing operations	169.6	134.8
Net income	$ 169.3	$ 134.6
Basic	$ 3.24	$ 2.57
Diluted	$ 3.19	$ 2.53
Basic Shares	52.4	52.5
Diluted Shares	53.2	53.2

Acquisitions Schedule of Intangible Assets Acquired as Part of Business Combination (Details) - ECS $ in Millions	Apr. 02, 2018 USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Identifiable intangible assets	$ 195.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	11 years
Trademarks
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 16.5
Contractual customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	144.6
Backlog
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	23.1
Non-compete agreements
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	10.3
Favorable contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 0.5

Acquisitions Business Acquisition, Pro Forma Revenue (Details) - ECS - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues	$ 3,548.9	$ 3,213.5
Income from continuing operations	169.6	134.8
Net income	$ 169.3	$ 134.6
Basic	$ 3.24	$ 2.57
Diluted	$ 3.19	$ 2.53
Basic Shares	52.4	52.5
Diluted Shares	53.2	53.2

Goodwill and Identifiable Assets Goodwill (Details) - USD ($) $ in Millions			12 Months Ended
	Oct. 17, 2019	Jan. 25, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Amortization of intangible assets			$ 51.1	$ 58.5	$ 33.4
Goodwill [Roll Forward]
Gross goodwill, period start			1,421.1	894.1
Goodwill, Acquired During Period				528.2
Translation adjustment			(0.3)	(1.2)
Gross goodwill, period end			1,486.9	1,421.1	894.1
Apex
Goodwill [Line Items]
Amortization of intangible assets			20.3	26.2	29.4
Goodwill [Roll Forward]
Gross goodwill, period start			(662.1)	662.1
Translation adjustment			0.0
Gross goodwill, period end			(703.5)	(662.1)	662.1
Oxford
Goodwill [Line Items]
Amortization of intangible assets			3.7	4.2	4.0
Goodwill [Roll Forward]
Gross goodwill, period start			(230.8)	232.0
Translation adjustment			(0.3)	(1.2)
Gross goodwill, period end			(230.5)	(230.8)	232.0
ECS
Goodwill [Line Items]
Amortization of intangible assets			27.1	28.1	0.0
Goodwill [Roll Forward]
Gross goodwill, period start			528.2	0.0
Gross goodwill, period end			552.9	528.2	$ 0.0
DHA
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 24.7	24.7
DHA \| ECS
Goodwill [Roll Forward]
Goodwill, Acquired During Period			24.7
Intersys
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 41.4		41.4
Intersys \| Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period			$ 41.4
ECS
Goodwill [Roll Forward]
Goodwill, Acquired During Period				$ 528.2

Goodwill and Identifiable Assets Acquired Intangible Assets (Details) - USD ($)
$ in Millions

12 Months Ended

Apr. 02, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Intangible Assets [Line Items]

Write-off Intangible Asset

$ 3.3

Amortization of intangible assets

51.1

$ 58.5

$ 33.4

Identifiable intangible assets, net

476.5

488.7

Intangible Assets, Gross (Excluding Goodwill)

783.6

745.7

Intangible assets subject to amortization:

Gross Carrying Amount

524.7

483.5

Accumulated Amortization

307.1

257.0

Net Carrying Amount

217.6

226.5

Intangible assets not subject to amortization:

Trademarks, Carrying Amount

258.9

^[1]

262.2

Contractual customer relationships

Intangible Assets [Line Items]

Estimated Useful Life

12 years 9 months

Intangible assets subject to amortization:

Gross Carrying Amount

384.9

346.9

Accumulated Amortization

179.9

145.4

Net Carrying Amount

$ 205.0

201.5

Contractual customer relationships | Minimum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

2 months

Contractual customer relationships | Maximum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

12 years 9 months

Contractor relations

Intangible assets subject to amortization:

Gross Carrying Amount

$ 71.1

71.1

Accumulated Amortization

70.6

67.1

Net Carrying Amount

$ 0.5

4.0

Contractor relations | Minimum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

2 years

Contractor relations | Maximum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

5 years

Backlog

Intangible Assets [Line Items]

Estimated Useful Life

2 years 9 months

Intangible assets subject to amortization:

Gross Carrying Amount

$ 25.0

23.1

Accumulated Amortization

23.9

17.7

Net Carrying Amount

$ 1.1

5.4

Backlog | Minimum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

1 year

Backlog | Maximum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

2 years 9 months

Non-compete agreements

Intangible assets subject to amortization:

Gross Carrying Amount

$ 24.8

22.1

Accumulated Amortization

13.8

9.9

Net Carrying Amount

$ 11.0

12.2

Non-compete agreements | Minimum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

4 years

2 years

Non-compete agreements | Maximum [Member]

Intangible Assets [Line Items]

Estimated Useful Life

7 years

In-use software

Intangible Assets [Line Items]

Estimated Useful Life

6 years

Intangible assets subject to amortization:

Gross Carrying Amount

$ 18.9

18.9

Accumulated Amortization

18.9

16.0

Net Carrying Amount

$ 0.0

2.9

Favorable contracts

Intangible Assets [Line Items]

Estimated Useful Life

5 years

Intangible assets subject to amortization:

Gross Carrying Amount

$ 0.0

1.4

Accumulated Amortization

0.0

0.9

Net Carrying Amount

$ 0.0

$ 0.5

[1]	Certain foreign trademarks totaling $3.3 million were written off during the second quarter of 2019.

Goodwill and Identifiable Assets Future Amortization Expense (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2020	$ 44.7
2021	37.8
2022	29.4
2023	25.4
2024	19.1
Thereafter	61.2
Net Carrying Amount	$ 217.6	$ 226.5

Property and Equipment (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment	$ 238.7	$ 204.5
Less -- accumulated depreciation	165.0	125.4
Total	73.7	79.1
Computer hardware and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	180.2	159.4
Furniture, fixtures and equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	26.8	22.2
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	24.7	19.2
Work-in-progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 7.0	$ 3.7

Property and Equipment (Narratives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Net book value	$ 73.7	$ 79.1
Depreciation	40.1	36.5	$ 25.2
Cost of Sales [Member]
Property, Plant and Equipment [Line Items]
Depreciation	10.3	7.7
Software Development
Property, Plant and Equipment [Line Items]
Net book value	34.8	41.3
Software Development Work-in-Progress
Property, Plant and Equipment [Line Items]
Net book value	$ 6.4	$ 3.1

Long-Term Debt (Details) $ in Millions	12 Months Ended
Long-Term Debt (Details) $ in Millions	Dec. 31, 2019 USD ($) Rate	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Senior Notes	$ 550.0	$ 0.0
Long-term Debt, Gross	1,040.8	1,124.0
Unamortized Debt Issuance Expense	(8.5)	(23.6)
Long-term Debt	1,032.3	1,100.4
Write-off of loan costs	$ 18.9	0.0	$ 0.0
Leverage ratio	2.30
FeesPaidInConnectionWithAmendment	$ 9.1
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	8.6
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense	0.5
$250 Million Revolving Credit Facility, Due November 2024 [Member]
Debt Instrument [Line Items]
Secured Debt	0.0	0.0
Line of Credit Facility, Maximum Borrowing Capacity	$ 250.0
Basis point reduction \| Rate	0.25%
$825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0	337.0
$822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Secured Debt	490.8	$ 787.0
Senior Notes
Debt Instrument [Line Items]
Face amount of term loan	$ 550.0
Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage \| Rate	4.625%
Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio \| Rate	4.50%
Leverage Ratio in Latest Year of Debt Term \| Rate	3.75%
Term Loan Facility \| Minimum [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings \| Rate	0.25%
Revolving credit facility, unused portion, commitment fee percentage \| Rate	0.20%
Term Loan Facility \| Minimum [Member] \| Eurodollar [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings \| Rate	1.25%
Term Loan Facility \| Maximum [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings \| Rate	1.25%
Revolving credit facility, unused portion, commitment fee percentage \| Rate	0.35%
Term Loan Facility \| Maximum [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate During Period \| Rate	3.55%
Term Loan Facility \| Maximum [Member] \| Eurodollar [Member] \| $822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings \| Rate	2.25%
Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 246.1

Commitments and Contingencies Commitments and Contingencies (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Self Insurance Reserve	$ 2.4	$ 2.4
Workers' compensation receivable	13.8	15.0
Letters of Credit Outstanding, Amount	$ 3.9	$ 4.4

Commitments and Contingencies Contractual Obligations (Details) $ in Millions	Dec. 31, 2019 USD ($)
Purchase Obligations
2019	$ 13.4
2020	11.6
2021	11.3
2022	4.4
2023	0.0
Thereafter	0.0
Total	$ 40.7

Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2019
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 250.0
Stock Repurchased and Retired During Period, Value	$ (20.0)		$ (58.1)
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	(0.3)	0.0	(1.1)
Stock Repurchased and Retired During Period, Value	$ 0.0		$ 0.0
Additional Paid-in Capital [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Value	(3.9)		(13.2)
Retained Earnings [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ (16.1)		$ (44.9)

Stock-based Compensation and Other Employee Benefit Plans RSU and RSAs (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	0.1
Award with service conditions [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested RSUs and RSAs outstanding at January 1 (in shares)	0.4
Granted (in shares)	0.2
Vested (in shares)	(0.1)
Forfeited (in shares)	0.0
Unvested RSUs and RSAs outstanding at December 31 (in shares)	0.5	0.4
Unvested and expected to vest RSUs and RSAs outstanding at December 31 (in shares)	0.4
Awards with performance and service conditions [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested RSUs and RSAs outstanding at January 1 (in shares)	0.9
Granted (in shares)	0.4
Vested (in shares)	(0.5)
Forfeited (in shares)	(0.1)
Unvested RSUs and RSAs outstanding at December 31 (in shares)	0.7	0.9
Unvested and expected to vest RSUs and RSAs outstanding at December 31 (in shares)	0.7
Restricted Stock Units and Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	2.9
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested RSUs and RSAs outstanding at January 1 (in shares)	1.3
Granted (in shares)	0.6
Vested (in shares)	(0.6)
Forfeited (in shares)	(0.1)
Unvested RSUs and RSAs outstanding at December 31 (in shares)	1.2	1.3
Unvested and expected to vest RSUs and RSAs outstanding at December 31 (in shares)	1.1
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Unvested RSUs and RSAs outstanding at January 1, weighted average grant date fair value (in dollars per share)	$ 60.87
Granted, weighted average grant date fair value (in dollars per share)	62.26	$ 74.61	$ 49.62
Vested, weighted average grant date fair value (in dollars per share)	56.58
Forfeited, weighted average grant date fair value (in dollars per share)	67.58
Unvested RSUs and RSAs outstanding at December 31, weighted average grant date fair value (in dollars per share)	63.21	$ 60.87
Unvested and expected to vest RSUs and RSAs outstanding at December 31, weighted average grant date fair value (in dollars per share)	$ 62.96
Liability Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Share-based Arrangements, Liability, Current and Noncurrent	$ 0.2	$ 0.5

Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Plan Assets	$ 11.8	$ 6.2
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	0.1
Share-based compensation	$ 39.3	31.5	$ 24.0
Share-based Payment Arrangement, Expense, Tax Benefit	$ 1.1	2.7	4.5
Restricted Stock Units and Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	2.9
Shares surrendered by the employees to the Company for payment of minimum tax withholding obligations	0.2
Total intrinsic value of options exercised	$ 38.7	16.4	$ 37.8
Unrecognized compensation expense	$ 38.5
Share-based awards expense, service period	2 years
Liability Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Share-based Arrangements, Liability, Current and Noncurrent	$ 0.2	$ 0.5

Stock-based Compensation and Other Employee Benefit Plans Employee Stock Purchase Plan (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	0.1
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date	15.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1.7
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 17.11	$ 15.09	$ 9.71
Stock Issued During Period, Shares, Employee Stock Purchase Plans	0.2	0.2	0.2
Share-based Payment Arrangement, Expense	$ 4.1	$ 2.7	$ 2.1
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	6 months	6 months	6 months
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate	0.00%	0.00%	0.00%
Minimum [Member] \| Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	23.00%	23.00%	23.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	0.45%	0.45%	0.45%
Maximum [Member] \| Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	38.50%	38.50%	38.50%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	2.37%	2.37%	2.37%

Stock-based Compensation and Other Employee Benefit Plans Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Deferred Compensation Plan Assets	$ 11.8	$ 6.2
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 16.2	$ 13.0	$ 7.5

Revenues (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Revenue from Contract with Customer [Abstract]
Contract with Customer, Refund Liability			$ 1.5
Deferred Revenue	$ 8.4	$ 9.8
Accounts Receivable, Allowance for Credit Loss, Current	$ 5.1	$ 4.8

Income Taxes (Narratives) (Details) - USD ($) $ in Millions	12 Months Ended
Income Taxes (Narratives) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
U.S. federal statutory tax rate	21.00%	21.00%	35.00%
Operating Loss Carryforwards, Valuation Allowance	$ 0.8	$ 1.0
Accumulated net earning	744.7	586.1
Gross deferred tax assets	58.3	32.5
Gross deferred tax liabilities	156.2	111.3
Unrecognized Tax Benefits	1.3	$ 0.4	$ 0.4	$ 1.3
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	0.0
Foreign Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	0.8
Accumulated net earning	$ 16.7

Income Taxes (Income Tax Components) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ 29.9	$ 20.6	$ 33.6
State	8.6	10.6	7.7
Foreign	4.6	3.9	2.8
Total Current	43.1	35.1	44.1
Deferred:
Federal & State	19.9	11.5	(4.7)
Foreign	(1.0)	(0.4)	(0.2)
Total Deferred	18.9	11.1	(4.9)
Total	$ 62.0	$ 46.2	$ 39.2

Income Taxes (Income Before Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ 218.7	$ 190.7	$ 184.2
Foreign	18.1	13.5	12.9
Income before income taxes	$ 236.8	$ 204.2	$ 197.1

Income Taxes (Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Intangibles	$ (112.7)	$ (90.8)
Depreciation expense	(13.3)	(15.0)
Deferred Tax Assets Leasing liabilities	25.7	0.0
Deferred Tax Liability Leasing Asset	(24.8)	0.0
Allowance for doubtful accounts	1.8	1.6
Employee related accruals	12.0	9.6
Stock-based compensation	9.2	9.5
Other	3.4	5.3
Net operating loss carryforwards - foreign	0.8	1.0
Valuation allowance	(0.8)	(1.0)
Total deferred income tax assets (liabilities)	$ (98.7)	$ (79.8)

Income Taxes (Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision at the statutory rate	$ 49.7	$ 42.9	$ 69.0
State income taxes, net of federal benefit	11.5	9.4	6.8
Permanent difference – non deductible items	1.7	1.6	1.9
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount	(0.9)	(2.2)	(4.2)
Work opportunity tax credit	(2.5)	(3.1)	(2.0)
Tax Adjustments, Settlements, and Unusual Provisions	0.0	(3.0)	(31.4)
Other	2.5	0.6	(0.9)
Total	$ 62.0	$ 46.2	$ 39.2

Income Taxes (Unrecognized Tax Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefit beginning of year	$ 0.4	$ 0.4	$ 1.3
Gross increases- tax positions in current year	0.0	0.0	0.1
Gross increases- tax positions in prior year	0.9	0.1	0.0
Gross decreases- tax positions in prior year	0.0	0.0	(0.3)
Reduction resulting from lapse of applicable statute of limitations	0.0	(0.1)	(0.7)
Unrecognized Tax Benefit end of year	$ 1.3	$ 0.4	$ 0.4

Earnings per Share (Details) - shares shares in Millions	12 Months Ended
Earnings per Share (Details) - shares shares in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted Average Number of Shares Outstanding Reconciliation
Weighted average number of common shares outstanding used to compute basic earnings per share	52.8	52.3	52.5
Dilutive effect of stock-based awards	0.6	0.8	0.7
Number of shares used to compute diluted earnings per share	53.4	53.1	53.2

Business Segments Segment Reporting Information (Details) - USD ($)
$ in Millions

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

^[1]

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Segment Reporting Information [Line Items]

Revenues

$ 1,025.2

$ 1,002.7

$ 972.3

$ 923.7

$ 929.7

$ 906.4

$ 878.5

$ 685.2

$ 3,923.9

$ 3,399.8

$ 2,626.0

Gross profit

$ 289.5

$ 291.4

$ 285.2

$ 263.9

$ 272.0

$ 270.1

$ 263.9

$ 217.7

1,130.0

1,023.7

850.1

Operating Income (Loss)

308.6

260.2

224.7

Amortization of intangible assets

51.1

58.5

33.4

Apex

Segment Reporting Information [Line Items]

Revenues

2,520.0

2,300.3

2,037.2

Gross profit

746.0

687.9

606.3

Operating Income (Loss)

287.7

262.4

222.0

Amortization of intangible assets

20.3

26.2

29.4

Oxford

Segment Reporting Information [Line Items]

Revenues

605.7

606.5

588.8

Gross profit

242.9

248.9

243.8

Operating Income (Loss)

48.4

54.1

53.8

Amortization of intangible assets

3.7

4.2

4.0

ECS

Segment Reporting Information [Line Items]

Revenues

798.2

493.0

0.0

Gross profit

141.1

86.9

0.0

Operating Income (Loss)

42.2

15.5

0.0

Amortization of intangible assets

27.1

28.1

0.0

Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual

798.2

493.0

0.0

Corporate

Segment Reporting Information [Line Items]

Operating Income (Loss)

^[2]

$ (69.7)

$ (71.8)

$ (51.1)

[1]	ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.
[2]	Corporate expenses primarily consist of consolidated stock-based compensation expense, compensation for corporate employees, acquisition, integration and strategic planning expenses, public company expenses and depreciation expense for corporate assets.

Business Segments Disaggregated Revenue (Details) - USD ($)
$ in Millions

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

^[1]

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Revenue from External Customer [Line Items]

Revenues

$ 1,025.2

$ 1,002.7

$ 972.3

$ 923.7

$ 929.7

$ 906.4

$ 878.5

$ 685.2

$ 3,923.9

$ 3,399.8

$ 2,626.0

Apex

Revenue from External Customer [Line Items]

Revenues

2,520.0

2,300.3

2,037.2

Apex | Assignment [Member]

Revenue from External Customer [Line Items]

Revenues

2,463.6

2,244.5

1,993.3

Apex | PermanentPlacement [Member]

Revenue from External Customer [Line Items]

Revenues

56.4

55.8

43.9

Oxford

Revenue from External Customer [Line Items]

Revenues

605.7

606.5

588.8

Oxford | Assignment [Member]

Revenue from External Customer [Line Items]

Revenues

522.7

516.0

503.1

Oxford | PermanentPlacement [Member]

Revenue from External Customer [Line Items]

Revenues

83.0

90.5

85.7

ECS

Revenue from External Customer [Line Items]

Revenues

798.2

493.0

0.0

ECS | Fixed-price Contract [Member]

Revenue from External Customer [Line Items]

Revenues

214.0

133.1

0.0

ECS | Time-and-materials Contract [Member]

Revenue from External Customer [Line Items]

Revenues

267.8

143.4

0.0

ECS | Cost-plus-fixed-fee Contract [Member]

Revenue from External Customer [Line Items]

Revenues

$ 316.4

$ 216.5

$ 0.0

[1]	ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

Business Segments ECS Segment Revenues by Customer Type (Details) - USD ($)
$ in Millions

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

^[1]

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Revenue from External Customer [Line Items]

Revenues

$ 1,025.2

$ 1,002.7

$ 972.3

$ 923.7

$ 929.7

$ 906.4

$ 878.5

$ 685.2

$ 3,923.9

$ 3,399.8

$ 2,626.0

ECS

Revenue from External Customer [Line Items]

Revenues

798.2

493.0

$ 0.0

ECS | department of defense and intelligence agencies [Member]

Revenue from External Customer [Line Items]

Revenues

453.9

311.0

ECS | federal civilian [Member]

Revenue from External Customer [Line Items]

Revenues

293.6

150.9

ECS | Commerical and Other [Member]

Revenue from External Customer [Line Items]

Revenues

$ 50.7

$ 31.1

[1]	ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

Business Segments Revenues by Geographic Location (Details) - USD ($)
$ in Millions

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

^[1]

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Revenues from External Customers and Long-Lived Assets [Line Items]

Revenues

$ 1,025.2

$ 1,002.7

$ 972.3

$ 923.7

$ 929.7

$ 906.4

$ 878.5

$ 685.2

$ 3,923.9

$ 3,399.8

$ 2,626.0

Domestic [Member]

Revenues from External Customers and Long-Lived Assets [Line Items]

Revenues

3,749.2

3,241.8

2,494.5

Foreign [Member]

Revenues from External Customers and Long-Lived Assets [Line Items]

Revenues

$ 174.7

$ 158.0

$ 131.5

[1]	ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

Business Segments Total Assets by Segment (Details) - USD ($)
$ in Millions

Dec. 31, 2019

Dec. 31, 2018

Segment Reporting, Asset Reconciling Item [Line Items]

Assets

$ 2,941.4

$ 2,687.8

Apex

Segment Reporting, Asset Reconciling Item [Line Items]

Assets

1,504.5

1,368.5

Oxford

Segment Reporting, Asset Reconciling Item [Line Items]

Assets

433.8

421.2

ECS

Segment Reporting, Asset Reconciling Item [Line Items]

Assets

935.2

865.6

Corporate

Segment Reporting, Asset Reconciling Item [Line Items]

Assets

^[1]

$ 67.9

$ 32.5

[1]

Corporate segment assets include workers’ compensation receivable and rabbi trust assets related to deferred compensation for employees of other segments. These programs are managed by the Company's corporate office and the related balances are recorded at corporate. These assets have offsetting liabilities for workers' compensation loss reserves and the Company's deferred compensation plan.

Business Segments Long-lived assets by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Asset Reconciling Item [Line Items]
Long-Lived Assets	$ 73.7	$ 79.1
Domestic [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Long-Lived Assets	71.4	76.7
Foreign [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Long-Lived Assets	$ 2.3	$ 2.4

Fair Value Measurements Fair Value Inputs, Liabilities, Quantitative Information (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Gross	$ 1,040.8	$ 1,124.0
Unamortized Debt Issuance Expense	(8.5)	(23.6)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	1,100.0
Deferred Compensation Plan Assets	$ 11.8	$ 6.2

Unaudited Quarterly Financial Results (Details) - USD ($)
$ / shares in Units, $ in Millions

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

^[1]

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Revenues

$ 1,025.2

$ 1,002.7

$ 972.3

$ 923.7

$ 929.7

$ 906.4

$ 878.5

$ 685.2

$ 3,923.9

$ 3,399.8

$ 2,626.0

Gross profit

289.5

291.4

285.2

263.9

272.0

270.1

263.9

217.7

1,130.0

1,023.7

850.1

Income from continuing operations

39.3

57.5

43.1

34.9

46.0

49.1

33.7

29.2

174.8

158.0

157.9

Income from discontinued operations, net of income taxes

(0.1)

(0.3)

(0.2)

Net income

$ 39.3

$ 57.4

$ 43.1

$ 34.9

$ 45.9

$ 49.1

$ 33.6

$ 29.1

$ 174.7

$ 157.7

Earnings per share:

Basic

$ 0.75

$ 1.09

$ 0.82

$ 0.66

$ 0.88

$ 0.94

$ 0.64

$ 0.56

$ 3.31

$ 3.02

$ 3.01

Diluted

$ 0.74

$ 1.08

$ 0.81

$ 0.66

$ 0.86

$ 0.93

$ 0.63

$ 0.55

$ 3.28

$ 2.98

$ 2.97

Discontinued Operations, Disposed of by Sale [Member]

Income from discontinued operations, net of income taxes

$ 0.0

$ (0.1)

$ 0.0

$ (0.1)

$ 0.0

$ (0.1)

$ (0.3)

[1]	ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

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- Definition

Amount after tax of income (loss) from a discontinued operation attributable to the parent. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

+ References + Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Event (Details) - BlackStone Federal - Subsequent Event [Member] $ in Millions	Jan. 24, 2020 USD ($)
Subsequent Event [Line Items]
Business Acquisition, Effective Date of Acquisition	Jan. 24, 2020
Business Acquisition, Name of Acquired Entity	Blackstone Technology Group
Payments to Acquire Businesses, Gross	$ 85.0
Business Acquisition, Description of Acquired Entity	headquartered in Arlington, Virginia
Business Combination, Reason for Business Combination	The acquisition expands the Company's capabilities in agile application development, cloud modernization and systems architecture, cybersecurity, user experience design and branding services to government clients

Schedule II - Valuation and Qualifying Accounts (Details) - USD ($)
$ in Millions

12 Months Ended

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Allowance for Doubtful Accounts and Bilinig Adjustments

SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]

Balance at beginning of year

$ 4.8

$ 9.9

$ 8.1

Provisions

3.7

3.3

^[1]

5.3

SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account

^[2]

0.0

6.8

Deductions

^[3]

(3.4)

(7.0)

(10.3)

Balance at end of year

5.1

4.8

9.9

Allowance for Workers' Compensation and Medical Malpractice Loss Reserves

SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]

Balance at beginning of year

17.4

14.8

15.8

Provisions

3.3

3.6

2.9

SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account

^[2]

0.0

Deductions

^[3]

(4.5)

(1.0)

(3.9)

Balance at end of year

$ 16.2

$ 17.4

$ 14.8

[1]	Upon adoption of ASC 606, permanent placement fallouts of $1.5 million were reclassified from allowance from doubtful accounts to other current liabilities, effective January 1, 2018.
[2]	Charges to other accounts include provision for permanent placement fallouts that have been deducted from net revenues and billing adjustments that have been deducted from net revenues in the accompanying consolidated statements of operations and comprehensive income.
[3]	Deductions from allowance for doubtful accounts include write-offs of uncollectible accounts receivable for the years ended December 31, 2019 and 2018. For the year ended December 31, 2017 deductions from allowance for doubtful accounts include write-offs of uncollectible accounts receivable, permanent placements fallouts that have been charged against the allowance for doubtful accounts and billing adjustments. Deductions from workers’ compensation loss reserves include payments of claims and changes related to anticipated insurance and indemnification recoveries for all periods presented.

Label

Element

Value

SEC Schedule, 12-09, Allowance, Credit Loss [Member]

SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount

us-gaap_ValuationAllowancesAndReservesBalance

$ 8,500,000

^[1]

[1]	Upon adoption of ASC 606, permanent placement fallouts of $1.5 million were reclassified from allowance from doubtful accounts to other current liabilities, effective January 1, 2018.