ASGN (ASGN) 10-K 1 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 20, 2019	Jun. 29, 2018
Entity Information [Line Items]
Entity Registrant Name	ASGN INCORPORATED
Entity Central Index Key	890,564
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		52,655,248
Entity Shell Company	false
Entity Public Float			$ 3,914,732,709
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and Cash Equivalents	$ 41,826	$ 36,667
Accounts receivable, net	613,825	428,536
Prepaid expenses and income taxes	11,446	18,592
Workers' compensation receivable	15,011	12,702
Other current assets	4,264	3,026
Total current assets	686,372	499,523
Property and equipment, net	79,123	57,996
Identifiable intangible assets, net	488,691	352,766
Goodwill	1,421,076	894,095
Other non-current assets	12,589	5,749
Total assets	2,687,851	1,810,129
Current liabilities:
Accounts payable	43,125	6,870
Accrued payroll and contract professional pay	194,807	114,832
Workers' compensation loss reserves	17,382	14,777
Income taxes payable	3,411	1,229
Other current liabilities	49,524	29,009
Total current liabilities	308,249	166,717
Long-term debt	1,100,424	575,213
Deferred income tax liabilities	79,833	69,436
Other long-term liabilities	17,283	7,372
Total liabilities	1,505,789	818,738
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred Stock, Value, Issued	0	0
Common Stock, Value, Issued	525	521
Paid-in capital	601,754	566,090
Retained earnings	586,125	428,419
Accumulated other comprehensive loss	(6,342)	(3,639)
Total stockholders’ equity	1,182,062	991,391
Total liabilities and stockholders’ equity	$ 2,687,851	$ 1,810,129

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Stockholders’ equity:
Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred Stock, shares authorized (in shares)	1,000,000	1,000,000
Preferred Stock, shares issued (in shares)	0	0
Preferred Stock, shares outstanding (in shares)	0	0
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (in shares)	75,000,000	75,000,000
Common Stock, shares issued (in shares)	52,511,440	52,151,538
Common Stock: shares outstanding (in shares)	52,511,440	52,151,538

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 3,399,781	$ 2,625,924	$ 2,440,413
Cost of services	2,376,130	1,775,851	1,645,230
Gross profit	1,023,651	850,073	795,183
Selling, general and administrative expenses	704,997	591,893	565,829
Amortization of intangible assets	58,506	33,444	39,628
Operating income	260,148	224,736	189,726
Interest expense	(55,973)	(27,643)	(32,327)
Income before income taxes	204,175	197,093	157,399
Provision for income taxes	46,191	39,219	60,203
Income from continuing operations	157,984	157,874	97,196
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(278)	(199)	5
Net income	$ 157,706	$ 157,675	$ 97,201
Earnings per share:
Basic	$ 3.02	$ 3.01	$ 1.83
Diluted	$ 2.98	$ 2.97	$ 1.81
Number of shares and share equivalents used to calculate earnings per share:
Basic (in shares)	52,333	52,503	53,192
Diluted (in shares)	53,061	53,205	53,747
Reconciliation of net income to comprehensive income:
Net income	$ 157,706	$ 157,675	$ 97,201
Foreign currency translation adjustment	(2,703)	6,384	(1,861)
Comprehensive income	$ 155,003	$ 164,059	$ 95,340

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance (in shares) at Dec. 31, 2015		53,024,972
Balance at Dec. 31, 2015	$ 784,794	$ 530	$ 542,859	$ 249,567	$ (8,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		397,182
Vesting of restricted stock units and restricted stock awards	(7,160)	$ 4	(7,164)
Employee stock purchase plan (in shares)		242,303
Employee stock purchase plan	7,507	$ 2	7,505
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		185,484
Exercise of common stock options	2,251	$ 2	2,249
Stock Repurchased and Retired During Period, Shares		(1,133,553)
Stock Repurchased and Retired During Period, Value	(43,091)	$ (11)	(11,885)	(31,195)
Tax deficiency from stock-based compensation	2,739		2,739
Stock-based compensation expense	26,559		26,559
Translation adjustments	(1,861)				(1,861)
Net income	97,201			97,201
Balance (in shares) at Dec. 31, 2016		52,716,388
Balance at Dec. 31, 2016	868,939	$ 527	562,862	315,573	(10,023)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		413,372
Vesting of restricted stock units and restricted stock awards	(15,801)	$ 4	(15,805)
Employee stock purchase plan (in shares)		214,466
Employee stock purchase plan	7,404	$ 2	7,402
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		36,561
Exercise of common stock options	351		351
Stock Repurchased and Retired During Period, Shares		(1,229,249)
Stock Repurchased and Retired During Period, Value	(58,070)	$ (12)	(13,229)	(44,829)
Stock-based compensation expense	24,509		24,509
Translation adjustments	6,384				6,384
Net income	$ 157,675			157,675
Balance (in shares) at Dec. 31, 2017	52,151,538	52,151,538
Balance at Dec. 31, 2017	$ 991,391	$ 521	566,090	428,419	(3,639)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		155,029
Vesting of restricted stock units and restricted stock awards	(4,695)	$ 2	(4,697)
Employee stock purchase plan (in shares)		160,407
Employee stock purchase plan	8,825	$ 2	8,823
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		44,466
Exercise of common stock options	547		547
Stock-based compensation expense	30,991		30,991
Translation adjustments	(2,703)				(2,703)
Net income	$ 157,706			157,706
Balance (in shares) at Dec. 31, 2018	52,511,440	52,511,440
Balance at Dec. 31, 2018	$ 1,182,062	$ 525	$ 601,754	$ 586,125	$ (6,342)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Cash Flows from Operating Activities:
Net income	$ 157,706	$ 157,675	$ 97,201
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	94,975	58,604	62,249
Provision for accounts receivable allowances	3,292	12,050	10,788
Provision (benefit) for deferred income taxes	11,215	(5,140)	12,834
Share-based compensation	31,488	24,044	27,024
Workers' Compensation provision	3,577	2,908	2,693
Other	(11,651)	(6,610)	(5,642)
Changes in operating assets and liabilities, net of effects of acquisitions:
Accounts receivable	(90,668)	(50,461)	(43,289)
Prepaid expenses and income taxes	(14,776)	11,861	(6,433)
Income taxes payable	2,135	613	3,346
Accounts payable	23,320	315	(2,236)
Accrued payroll and contract professional pay	25,427	1,040	23,556
Workers' compensation loss reserve	(3,282)	(2,616)	(2,691)
Payments of earn-outs	0	0	(4,780)
Other	1,839	2,665	561
Net cash provided by operating activities	287,451	196,446	199,331
Cash Flows from Investing Activities:
Cash paid for property and equipment	(28,701)	(24,265)	(27,138)
Cash paid for acquisitions, net of cash acquired	(760,216)	(25,940)	0
Cash received from sale of discontinued operations, net	0	0	6,000
Other	184	88	(846)
Net cash used in investing activities	(788,733)	(50,117)	(21,984)
Cash Flows from Financing Activities:
Principal payments of long-term debt	(286,000)	(105,000)	(129,000)
Proceeds from long-term debt	822,000	37,000	11,000
Proceeds from option exercises and employee stock purchase plan	9,372	7,755	9,758
Payment of employment taxes related to release of restricted stock awards	(5,649)	(14,886)	(7,024)
Repurchase of common stock	0	(60,065)	(41,096)
Debt issuance or amendment costs	(22,450)	(3,273)	(889)
Payments of earn-outs	0	0	16,814
Other	(9,474)	0	0
Net cash provided by (used in) financing activities	507,799	(138,469)	(174,065)
Effect of exchange rate changes on cash and cash equivalents	(1,358)	1,763	(107)
Cash and Cash Equivalents, Period Increase (Decrease)	5,159	9,623	3,175
Cash and Cash Equivalents	41,826	36,667	27,044
Supplemental Disclosure of Cash Flow Information
Income taxes	21,399	55,326	38,190
Interest	50,966	24,074	26,829
Non-Cash Investing and Financing Activities:
Unpaid portion of additions to property and equipment	2,466	2,313	438
Unsettled repurchases of common stock	$ 0	$ 0	$ 1,995

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
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 10 – 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. Office space leases range from six months to 11 years . Rent expense is recognized on a straight-line basis over the lease term. Differences between the straight-line rent expense and amounts payable under the leases are recorded as deferred rent which is included in other current liabilities and other long-term liabilities, as appropriate, in the consolidated balance sheets. This includes the impact of both scheduled rent increases and free or reduced periods. Allowances provided by landlords for leasehold improvements are included in deferred rent, 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 st for each of its four reporting units. When evaluating goodwill for impairment, the Company may first perform a qualitative assessment (“step zero” of the impairment test) 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 (“step one” of the impairment test) 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 our 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 2018 , 2017 and 2016 . 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, both per occurrence and in the aggregate. 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 our 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 our ECS segment are primarily from the U.S. government and are considered to have low credit risk.

Accounting Standards Update

Accounting Standards Update	12 Months Ended
	Dec. 31, 2018
Accounting Changes and Error Corrections [Abstract]
Accounting Standards Update	Accounting Standards Update Recently Adopted Accounting Pronouncements Effective January 1, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2014-09, Revenue from Contracts with Customers (Topic 606) , using the modified retrospective method. This update outlined a comprehensive new revenue recognition model designed to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The new standard also required additional quantitative and qualitative disclosures (see Note 10 – Revenues ). Accounting Pronouncements Not Yet Adopted 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, using the transition relief and will present the effects of this standard prospectively. The Company will recognize and measure the right of use asset and lease liability from operating leases on the consolidated balance sheet without revising comparative period information or disclosures. The Company elected the package of practical expedients which allowed the Company to forgo the reassessment of (i) whether contracts are or contain leases, (ii) classification of leases and (iii) initial direct costs of existing leases. The Company also elected the practical expedient to not separate non-lease components. At the transition date, the estimated right of use asset and lease liability are approximately $95.0 and $100.0 million , respectively. The difference between the right of use asset and lease liability is due to the derecognition of deferred rent and prepaid rent. The adoption of the standard did not have an effect on the Company’s stockholders' equity and is not expected to have an effect on the Company’s results of operations and cash flows. The adoption of the new standard will result in additional disclosures, including information on variable lease costs and short-term lease costs. In January 2017, the FASB issued 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 Company is required to adopt this standard on January 1, 2020, and early adoption is permitted. The Company does not expect the adoption of this guidance will have a material impact on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement , which modifies the disclosure requirements on fair value measurements. The amendments in this update are effective for interim and annual periods for the Company beginning on January 1, 2020, with early adoption permitted. The Company is evaluating the effects that the adoption of this guidance will have on its disclosures. In August 2018, the FASB issued 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 for the Company beginning on January 1, 2020, with early adoption permitted. The amendments in this update may be applied either retrospectively or prospectively. The Company is in the process of evaluating the impact the standard will have on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606. The amendments in this update provide guidance on how to assess whether certain transactions between collaborative arrangement participants should be accounted for within the revenue recognition standard. The amendments in this update are effective for interim and annual periods for the Company beginning on January 1, 2020, with early adoption permitted. The Company is in the process of evaluating the impact the standard will have on its consolidated financial statements. In August 2018, the SEC adopted the final rule under 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. Under the amendments, an analysis of changes in each caption of stockholders' equity presented in the balance sheet at interim reporting periods must be provided in a note or separate statement. The analysis should present a reconciliation of the beginning balance to the ending balance of each period for which a statement of comprehensive income is presented. The Company plans to include this presentation of interim changes in stockholders' equity in its Quarterly Report on Form 10-Q for the quarter ending March 31, 2019.

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions 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 have been included in the consolidated results of the Company from the date of 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 (a non-recurring fair value measurement based on 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 consider ation paid and the fair value of assets acquired and liabilities assumed (in thousands): Cash $ 12,400 Accounts receivable 97,167 Prepaid expenses and other current assets 8,568 Property and equipment 28,977 Identifiable intangible assets 194,950 Goodwill 528,207 Other non-current assets 1,282 Total assets acquired 871,551 Current liabilities 94,667 Long-term liabilities 4,268 Total liabilities assumed 98,935 Total purchase price $ 772,616 The following table summarizes the acquired identifiable intangible assets of ECS (in thousands): Useful life Contractual customer relationships 12.75 years $ 144,600 Backlog 2.75 years 23,100 Non-compete agreements 4 to 7 years 10,250 Favorable contracts 5 years 500 Trademarks indefinite 16,500 Total identifiable intangible assets acquired $ 194,950 The weighted-average amortization period for identifiable intangible assets, excluding trademarks, is 11 years . The summary below (in thousands, 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 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 $ 3,548,820 $ 3,213,465 Income from continuing operations $ 169,604 $ 134,830 Net income $ 169,326 $ 134,632 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 52,353 52,503 Diluted 53,164 53,245 Stratacuity Acquisition On August 8, 2017 , the Company acquired all of the outstanding shares of StratAcuity Staffing Partners, Inc. ("Stratacuity") headquartered in New Hampshire for $25.9 million . Acquisition expenses of approximately $0.5 million were included in SG&A expenses in 2017. Stratacuity was purchased to expand the Company's specialized clinical/scientific staffing solutions for biotechnology and pharmaceutical organizations . Goodwill associated with this acquisition totaled $17.5 million and is deductible for income tax purposes. Identifiable intangible assets related to this acquisition totaled $7.6 million . The results of operations for this acquisition have been combined with those of the Company from the acquisition date. Revenues and income before income taxes from Stratacuity of $7.5 million and $0.6 million , respectively, were included in the consolidated statement of operations and comprehensive income for 2017. The purchase accounting for this acquisition has been finalized. Stratacuity is included in the Apex segment.

Goodwill and Identifiable Asset

Goodwill and Identifiable Assets	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Identifiable Intangible Assets	Goodwill and Other Identifiable Intangible Assets The changes in the carrying amount of goodwill for the years ended December 31, 2018 and 2017 are as follows (in thousands): Apex Segment Oxford Segment ECS Segment Total Balance as of December 31, 2016 $ 644,617 $ 228,896 $ — $ 873,513 Stratacuity acquisition 17,467 — — 17,467 Translation adjustment — 3,115 — 3,115 Balance as of December 31, 2017 662,084 232,011 — 894,095 ECS acquisition — — 528,207 528,207 Translation adjustment — (1,226 ) — (1,226 ) Balance as of December 31, 2018 $ 662,084 $ 230,785 $ 528,207 $ 1,421,076 As of December 31, 2018 and 2017 , the Company had the following acquired intangible assets (in thousands): 2018 2017 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 $ 346,799 $ 145,424 $ 201,375 $ 202,588 $ 119,272 $ 83,316 Contractor relationships 2 - 5 years 71,104 67,072 4,032 71,121 59,174 11,947 Backlog 2.75 years 23,100 17,740 5,360 — — — Non-compete agreements 2 - 7 years 22,128 9,907 12,221 11,850 6,600 5,250 In-use software 6 years 18,900 15,966 2,934 18,900 12,816 6,084 Favorable contracts 5 years 1,400 853 547 900 673 227 483,431 256,962 226,469 305,359 198,535 106,824 Not subject to amortization: Trademarks 262,222 — 262,222 245,942 — 245,942 Total $ 745,653 $ 256,962 $ 488,691 $ 551,301 $ 198,535 $ 352,766 Amortization expense for intangible assets with finite lives w as $58.5 million in 2018 , $33.4 million in 2017 and $39.6 million in 2016 . Estimated amortization for each of the next five years and thereafter follows (in thousands): 2019 $ 47,280 2020 36,401 2021 30,705 2022 22,970 2023 19,622 Thereafter 69,491 $ 226,469

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment.	Property and Equipment Property and equipment at December 31, 2018 and 2017 consisted of the following (in thousands): 2018 2017 Computer hardware and software $ 159,357 $ 115,444 Furniture, fixtures and equipment 22,242 17,556 Leasehold improvements 19,229 16,628 Work-in-progress 3,690 4,565 204,518 154,193 Less -- accumulated depreciation (125,395 ) (96,197 ) $ 79,123 $ 57,996 The Company has capitalized costs related to its various technology initiatives. At December 31, 2018 and 2017 , the net book value of the property and equipment related to computer software was $41.3 million and $33.3 million , respectively, which included work-in-progress of $3.1 million and $3.2 million , respectively. Depreciation expense related to property and equipment was $36.5 million in 2018 , $25.2 million in 2017 and $22.6 million in 2016 and is included in SG&A expenses and costs of services ( $7.7 million in costs of services in 2018, related to ECS).

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Long-Term Debt	Long-Term Debt At December 31, 2018 and 2017 , long-term debt consisted of the following (in thousands): 2018 2017 $200 million revolving credit facility, due March 31, 2023 $ — $ — Term B loan facility, due June 5, 2022 337,000 588,000 Term B loan facility, due April 2, 2025 787,000 — 1,124,000 588,000 Unamortized deferred loan costs (23,576 ) (12,787 ) $ 1,100,424 $ 575,213 On April 2, 2018, in connection with the acquisition of ECS, the Company amended its credit facility mainly to add an $822.0 million tranche to the term B loan facility that matures on April 2, 2025. The amendment also provided for the ability to increase the loan facilities by an amount not to exceed the sum of (i) $300.0 million , (ii) the aggregate principal of voluntary prepayments of the term B loans and permanent reductions of the revolving commitments, and (iii) additional amounts so long as the pro forma consolidated secured leverage ratio is no greater than 3.25 to 1.00. The revolving credit facility was also amended to extend the maturity date to March 31, 2023. The Company incurred $22.5 million of debt issuance and amendment costs, of which $15.3 million are presented in the consolidated balance sheet as a reduction of outstanding debt and are being amortized over the term of the credit facility, $6.2 million were expensed as incurred and presented in interest expense, and the remaining fees were presented in other current assets and other non-current assets and are being amortized over the term of the credit facility. Borrowings under the term B loans bear interest at LIBOR, plus 2.00 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. At December 31, 2018 , the interest on the term B loans was 4.52 percent and there were no borrowings under the revolving credit facility. For the term B loan that matures on June 5, 2022, there are no required minimum payments until its maturity date. For the term B loan that matures on April 2, 2025, the Company is required to make minimum quarterly payments of $2.1 million ; however, as a result of principal payments made through December 31, 2018 , the first required minimum quarterly payment of $2.1 million is not due until September 30, 2022 . The Company is also 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 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 earnings before interest, taxes, depreciation and amortization ("EBITDA"), which steps down at regular intervals from 4.75 to 1.00 as of December 31, 2018 , to 3.75 to 1.00 as of September 30, 2021 and thereafter. The 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, 2018 , the Company was in compliance with its debt covenants, its ratio of consolidated secured debt to consolidated EBITDA was 2.69 to 1.00, and it had $195.6 million available borrowing capacity under its revolving credit facility.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Company leases its facilities under operating leases, which expire at various dates through 2027. Certain leases contain rent escalations or renewal options or both. Rent expense for all significant leases is recognized on a straight-line basis. Rent expense is included in SG&A expenses and was $32.7 million , $26.4 million and $25.6 million in 2018 , 2017 and 2016 , respectively. The balance of the deferred rent liability reflected in other current liabilities in the accompanying consolidated balance sheets was $1.3 million and $1.2 million at December 31, 2018 and 2017 , respectively, and the balance reflected in other long-term liabilities was $5.6 million and $5.0 million , at December 31, 2018 and 2017 , respectively. The following is a summary of the Company's specified contractual cash obligations as of December 31, 2018 , excluding current liabilities that are included in the consolidated balance sheet (in thousands): Operating Leases Related Party Leases Purchase Obligations Total 2019 $ 29,664 $ 1,285 $ 23,162 $ 54,111 2020 24,103 1,317 5,424 30,844 2021 19,762 1,350 3,615 24,727 2022 14,262 1,383 — 15,645 2023 8,859 1,418 — 10,277 Thereafter 6,285 1,082 — 7,367 Total $ 102,935 $ 7,835 $ 32,201 $ 142,971 The Company leases two propertie s owned by certain board members and an executive of the Company. Rent expense for these two properties was $1.3 million for each of the years 2018 , 2017 and 2016 . 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 $2.1 million , net of anticipated insurance and indemnification recoveries of $15.0 million and $12.7 million , at December 31, 2018 and 2017 , respectively. The Company has unused stand-by letters of credit outstanding to secure obligations for workers’ compensation claims with various insurance carriers. The unused stand-by letters of credit at December 31, 2018 and 2017 were $4.4 million . Certain employees participate in the Company’s Amended and Restated Change in Control Severance Plan, or have separate agreements that provide for certain benefits in the event of termination at the Company's convenience or following a change in control, 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

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Treasury Stock [Text Block]	Stockholders' Equity On June 10, 2016, the Board of Directors approved a $150.0 million , two-year stock repurchase program. Under this program, the Company repurchased 1.2 million shares of its common stock in 2017 at a cost of $58.1 million and 1.1 million shares in 2016 at a cost of $43.1 million . All shares repurchased were retired, which resulted in a reduction in paid-in capital of $13.2 million in 2017 and $11.9 million in 2016 and a reduction in retained earnings of $44.8 million in 2017 and $31.2 million in 2016 . The balances of accumulated other comprehensive income (loss) at December 31, 2018 , 2017 and 2016 and the activity within those years was comprised of foreign currency translation adjustments.

Stock-based Compensation and Ot

Stock-based Compensation and Other Employee Benefit Plans	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan	Stock-Based Compensation and Other Employee Benefit Plans The stockholders of the Company approved the adoption of the Company’s 2010 Incentive Award Plan effective as of June 3, 2010, which plan has been amended from time to time (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, 2018 , there were 927,572 shares available for issuance under the 2010 Plan. The Board of Directors adopted the Company’s 2012 Employment Inducement Incentive Award Plan on May 15, 2012, which plan has been amended from time to time (the “2012 Plan”). This plan allows for grants of stock to employees as employment inducement awards pursuant to New York Stock Exchange rules. The terms of the 2012 Plan are similar to the 2010 Plan. As of December 31, 2018 , there were 148,434 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 was $31.5 million , $24.0 million and $27.0 million for the years ended December 31, 2018 , 2017 and 2016 , respectively, and is included in SG&A expenses. The Company recognized $2.7 million , $4.5 million and $2.7 million of excess tax benefits in 2018 , 2017 and 2016 . The 2018 and 2017 excess tax benefits are recorded in the consolidated statements of operations and comprehensive income. The 2016 excess tax benefits are presented in the consolidated statement of stockholders' equity, as this was the presentation prior to the adoption of ASU No. 2016-09 Compensation - Stock Compensation (Topic 718). 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, 2018 and changes during the year then ended are presented below: Service Conditions Performance and Service Conditions Total Weighted Average Grant-Date Fair Value Per Unit Unvested RSUs outstanding at December 31, 2017 207,771 652,202 859,973 $ 44.29 Granted 343,912 389,480 733,392 $ 74.61 Vested (110,500 ) (107,941 ) (218,441 ) $ 43.71 Forfeited (16,502 ) (40,475 ) (56,977 ) $ 53.23 Unvested RSUs outstanding at December 31, 2018 424,681 893,266 1,317,947 $ 60.87 Unvested and expected to vest RSUs outstanding at December 31, 2018 361,250 859,007 1,220,257 $ 60.49 The total number of shares vested in the table above includes 63,412 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 2018 , 2017 and 2016 was $74.61 , $49.62 and $36.07 per award, respectively. The fair value of RSUs that vested during 2018 , 2017 and 2016 , was $16.4 million , $37.8 million and $21.7 million , respectively. As of December 31, 2018 , there was unrecognized compensation expense of $41.1 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. This liability is included in other accrued expenses in the accompanying consolidated balance sheets . The following table summarizes the balance of liability awards, expense and settlements (for vested awards) during the years presented (in thousands): 2018 2017 2016 Liability awards beginning of year $ — $ 465 $ — Expense for grants 503 470 465 Settled (6 ) (935 ) — Liability awards end of year $ 497 $ — $ 465 There was no unrecognized compensation expense for liability awards as of December 31, 2018 . Stock Options The Company has not granted any stock options since 2012. The fair value of stock option grants was estimated on the grant date using the Black-Scholes option pricing model. The Company records compensation expense based on this value. The following summarizes pricing and term information for options outstanding as of December 31, 2018 : Options Outstanding Options Exercisable Number Outstanding at Weighted Average Remaining Contractual Life (years) Weighted Average Exercise Price Number Exercisable at Weighted Average Exercise Price Range of Exercise Prices December 31, 2018 December 31, 2018 $ 6.89 - $ 7.31 12,936 1.1 $ 7.09 12,936 $ 7.09 $ 7.39 - $ 7.39 3,500 1.3 $ 7.39 3,500 $ 7.39 $ 8.26 - $ 8.26 9,256 2.1 $ 8.26 9,256 $ 8.26 $ 10.46 - $ 10.46 12,170 2.9 $ 10.46 12,170 $ 10.46 $ 16.51 - $ 16.51 50,000 3.7 $ 16.51 50,000 $ 16.51 $ 6.89 - $ 16.51 87,862 2.9 $ 13.05 87,862 $ 13.05 The following table is a summary of stock option activity during 2018 : Non- Qualified Stock Options Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value Outstanding at December 31, 2017 132,670 $ 12.79 3.8 $ 6,830,000 Exercised (44,466 ) $ 12.32 Canceled (342 ) $ 5.18 Outstanding at December 31, 2018 87,862 $ 13.05 2.9 $ 3,642,000 Vested and Expected to Vest at December 31, 2018 87,862 $ 13.05 2.9 $ 3,642,000 Exercisable at December 31, 2018 87,862 $ 13.05 2.9 $ 3,642,000 The total intrinsic value of options exercised during 2018 , 2017 and 2016 was $3.2 million , $1.5 million and $4.7 million , respectively. 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, 2018 , there were 1,925,448 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 (dollars in thousands, except per share amounts): Year Ended December 31, Average Black-Scholes Valuation per Share Shares Expense 2018 $15.09 160,407 $ 2,740 2017 $9.71 214,466 $ 2,092 2016 $9.05 242,303 $ 2,497 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 $6.2 million at December 31, 2018 , and was included in other long-term liabilities. The Company established a rabbi trust to fund the deferred compensation plan (see Note 14 – 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 $13.0 million , $7.5 million and $5.6 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. The increase in matching contributions to the 401(k) plans in 2018 was primarily due to the ECS acquisition.

Revenues

Revenues	12 Months Ended
	Dec. 31, 2018
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 13 – 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 3 – 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-plus-fixed-fee ("CPFF") contracts provide for reimbursement of direct contract costs and allowable and allocable indirect costs, plus a negotiated profit margin or fee. CPFF 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 CPFF 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 $0.6 million at January 1, 2018 and $9.8 million at December 31, 2018 ; the increase in contract liabilities was due to ECS, which at the acquisition date had contract liabilities of $11.6 million . Contract liabilities are included in other current liabilities on the consolidated balance sheet. During the current year the Company recognized revenues of $11.5 million relating to amounts that were included in contract liabilities as of the ECS acquisition date. The remaining balance at December 31, 2018 is related to new deferred revenue arrangements, primarily from ECS. 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 $4.8 million at December 31, 2018 and $9.9 million at December 31, 2017. The allowance at December 31, 2017 included $3.1 million for estimated permanent placement fallouts and various billing adjustments. Beginning in 2018, with the adoption of ASC 606, the permanent placement fallouts are now included in other current liabilities in the accompanying consolidated balance sheet and billing adjustments are a direct reduction of the gross accounts receivable balance.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The U.S. Tax Cuts and Jobs Act ("TCJA") was signed into law on December 22, 2017. The TCJA significantly revised the U.S. corporate income tax by, among other things, lowering the statutory corporate tax rate from 35 percent to 21 percent , eliminating certain deductions, imposing a mandatory one-time tax on accumulated earnings of foreign subsidiaries, introducing new tax regimes, and changing how foreign earnings are subject to U.S. tax. The TCJA also enhanced and extended the option to claim accelerated depreciation deductions by allowing full expensing of qualified property, primarily equipment, through 2022. The Company reasonably estimated the effects of the TCJA and recorded provisional amounts in its financial statements as of December 31, 2017. In 2017, the Company recorded a provisional tax benefit for the impact of the TCJA of approximately $31.4 million . This amount was primarily comprised of the re-measurement of federal net deferred tax liabilities resulting from the permanent reduction in the U.S. statutory corporate tax rate to 21 percent from 35 percent , after taking into account the mandatory one-time tax on the accumulated earnings of our foreign subsidiaries. The amount of this one-time tax was not material. In 2018, the Company recorded an additional $3.0 million tax benefit and completed its determination of the accounting implications of the TCJA. The provision for income taxes consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 Current: Federal $ 20,550 $ 33,623 $ 38,103 State 10,617 7,717 6,685 Foreign 3,896 2,725 2,672 35,063 44,065 47,460 Deferred: Federal and State 11,471 (4,726 ) 13,169 Foreign (343 ) (120 ) (426 ) 11,128 (4,846 ) 12,743 $ 46,191 $ 39,219 $ 60,203 Income from continuing operations before income taxes consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 United States $ 190,688 $ 184,168 $ 148,402 Foreign 13,487 12,925 8,997 $ 204,175 $ 197,093 $ 157,399 The components of deferred tax assets (liabilities) are as follows (in thousands): December 31, 2018 2017 Intangibles $ (90,834 ) $ (73,921 ) Depreciation expense (14,983 ) (9,851 ) Allowance for doubtful accounts 1,600 2,615 Employee-related accruals 9,608 2,616 Stock-based compensation 9,506 4,766 Other 5,270 4,339 Net operating loss carryforwards–foreign 999 1,255 Valuation allowance (999 ) (1,255 ) $ (79,833 ) $ (69,436 ) The reconciliation between the amount computed by applying the U.S. federal statutory tax rate of 21 percent in 2018 and 35 percent in 2017 and 2016 to income before income taxes, for each respective year, and the income tax provision is as follows (in thousands): Year Ended December 31, 2018 2017 2016 Income tax provision at the statutory rate $ 42,877 $ 68,983 $ 55,089 State income taxes, net of federal benefit 9,432 6,751 4,873 Disallowed meals and entertainment expenses 1,591 1,854 1,814 Excess stock-based compensation benefit (2,193 ) (4,241 ) (165 ) Work opportunity tax credit (3,081 ) (1,950 ) (1,463 ) Impact of TCJA (2,992 ) (31,420 ) — Other 557 (758 ) 55 $ 46,191 $ 39,219 $ 60,203 As of December 31, 2018 , the Company had no federal net operating losses, no state net operating losses, and $1.0 million of foreign net operating losses which begin to expire in 2021. The foreign net operating losses in the United Kingdom and Spain can be carried forward indefinitely. The Company has recorded a valuation allowance of approximately $1.0 million and $1.2 million at December 31, 2018 and 2017 , related to net operating loss carryforwards. The Company has not provided deferred income taxes on undistributed earnings and basis differences of its foreign subsidiaries as it intends to reinvest these earnings indefinitely. At December 31, 2018 , undistributed earnings of foreign subsidiaries were $15.5 million . Upon repatriation of those earnings, in the form of dividends or otherwise, the Company would be subject to state income and foreign withholding taxes. Determination of the amount of unrecognized deferred income tax liability is not practicable due to the complexities associated with this hypothetical calculation. The Company will remit non-indefinitely reinvested earnings where excess cash has accumulated and the Company determines that it is advantageous for business operations, tax or cash management reasons. The Company had gross deferred tax assets of $32.5 million and $18.6 million and gross deferred tax liabilities of $111.3 million and $86.8 million at December 31, 2018 and 2017 , respectively. Management has determined the gross deferred tax assets are realizable, with the exception of foreign net operating losses discussed above. At December 31, 2018 , 2017 and 2016 , there were $0.4 million , $0.4 million and $1.3 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 2019. The following is a reconciliation of the total amounts of unrecognized tax benefits (in thousands): Year Ended December 31, 2018 2017 2016 Unrecognized tax benefit beginning of year $ 430 $ 1,293 $ 814 Gross increases - tax positions in current year 20 71 — Gross increases - tax positions in prior year 69 — 479 Gross decreases - tax positions in prior year — (254 ) — Lapse of the statute of limitations (130 ) (680 ) — Unrecognized tax benefit end of year $ 389 $ 430 $ 1,293 The Company is subject to taxation in the United States and various states and foreign jurisdictions. The IRS has completed an examination of the Company's U.S. income tax return for 2014, resulting in no changes. The Company remains subject to U.S. federal income tax examinations for 2015 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 2014 and subsequent years.

Earnings per Share

Earnings per Share	12 Months Ended
	Dec. 31, 2018
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 thousands): Year Ended December 31, 2018 2017 2016 Weighted average number of common shares outstanding used to compute basic earnings per share 52,333 52,503 53,192 Dilutive effect of stock-based awards 728 702 555 Number of shares used to compute diluted earnings per share 53,061 53,205 53,747 There were no significant share equivalents outstanding as of December 31, 2018 , 2017 and 2016 that were anti-dilutive when applying the treasury stock method.

Business Segments

Business Segments	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment Reporting Disclosure [Text Block]	Business Segments ASGN is one of the foremost providers of IT and professional staffing services in the technology, digital, creative, engineering and life sciences fields across commercial and government sectors. Operating through its Apex, Oxford and ECS segments. The Apex Segment provides technical, scientific, digital and creative services and solutions to Fortune 1000 and mid-market clients across the United States and Canada. The businesses in this segment include Apex Systems, Apex Life Sciences and Creative Circle. The Oxford Segment provides hard-to-find technical, digital, engineering and life sciences resources and consulting services in select skill and geographic markets. The businesses in this segment include Oxford, CyberCoders and Life Sciences Europe. The ECS Segment 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. ECS was acquired on April 2, 2018 (see Note 3 – Acquisitions ). The Company’s management 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 thousands): Year ended December 31, 2018 2017 2016 Apex: Revenues $ 2,300,242 $ 2,037,154 $ 1,836,488 Gross profit 687,861 606,294 548,421 Operating income 262,297 222,010 195,133 Amortization 26,185 29,361 34,359 Oxford: Revenues $ 606,510 $ 588,770 $ 603,925 Gross profit 248,875 243,779 246,762 Operating income 54,142 53,787 51,294 Amortization 4,179 4,083 5,269 ECS: Revenues $ 493,029 $ — $ — Gross profit 86,915 — — Operating income 15,514 — — Amortization 28,142 — — Corporate: Operating loss (1) $ (71,805 ) $ (51,061 ) $ (56,701 ) Consolidated: Revenues $ 3,399,781 $ 2,625,924 $ 2,440,413 Gross profit 1,023,651 850,073 795,183 Operating income 260,148 224,736 189,726 Amortization 58,506 33,444 39,628 _________ (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 thousands): Year Ended December 31, 2018 2017 2016 Apex: Assignment $ 2,244,470 $ 1,993,256 $ 1,791,566 Permanent placement 55,772 43,898 44,922 $ 2,300,242 $ 2,037,154 $ 1,836,488 Oxford: Assignment $ 516,045 $ 503,058 $ 520,705 Permanent placement 90,465 85,712 83,220 $ 606,510 $ 588,770 $ 603,925 ECS: Firm-fixed-price $ 133,146 $ — $ — Time and Materials 143,434 — — Cost-plus-fixed-fee 216,449 — — $ 493,029 $ — $ — Consolidated $ 3,399,781 $ 2,625,924 $ 2,440,413 The following table presents the ECS segment revenues by customer type (in thousands): Year Ended December 31, 2018 Department of Defense and Intelligence Agencies $ 310,984 Federal Civilian 150,922 Commercial and Other 31,123 $ 493,029 The Company operates internationally, with operations in the United States, Europe and Canada. The following table presents revenues by geographic location (in thousands): Year Ended December 31, 2018 2017 2016 Domestic $ 3,241,804 $ 2,494,446 $ 2,324,713 Foreign 157,977 131,478 115,700 $ 3,399,781 $ 2,625,924 $ 2,440,413 The following table presents total assets by reportable segment (in thousands): December 31, 2018 2017 Apex $ 1,368,548 $ 1,357,246 Oxford 421,199 424,593 ECS 865,610 — Corporate (1) 32,494 28,290 $ 2,687,851 $ 1,810,129 _________ (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 thousands): December 31, 2018 2017 Domestic $ 76,742 $ 56,137 Foreign 2,381 1,859 $ 79,123 $ 57,996

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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, 2018 was $1.1 billion , excluding the $23.6 million of unamortized deferred loan costs (see Note 6 – Long-Term Debt ). The fair value of the term B loans was $1.1 billion as of December 31, 2018 and was determined using Level 1 inputs (quoted prices in active markets for identical assets and liabilities) from the fair value hierarchy. The Company had investments, primarily mutual funds, of $6.2 million at December 31, 2018 , held in a rabbi trust restricted to fund the Company's deferred compensation plan. The fair value of these investments was determined using Level 1 inputs from the fair value hierarchy. These assets are 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). For 2018 and 2017 , no fair value adjustments were required for non-financial assets or liabilities.

Unaudited Quarterly Financial R

Unaudited Quarterly Financial Results	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Result	Unaudited Quarterly Results The following tables present unaudited quarterly financial information (in thousands, 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, 2018 Mar. 31, June 30, (1) Sept. 30, Dec. 31, Revenues $ 685,173 $ 878,509 $ 906,449 $ 929,650 $ 3,399,781 Gross profit 217,737 263,846 270,172 271,896 1,023,651 Income from continuing operations 29,240 33,601 49,217 45,926 157,984 Loss from discontinued operations, net of income taxes (148 ) (40 ) (45 ) (45 ) (278 ) Net income $ 29,092 $ 33,561 $ 49,172 $ 45,881 $ 157,706 Per share income from continuing operations and net income: Basic $ 0.56 $ 0.64 $ 0.94 $ 0.88 $ 3.02 Diluted $ 0.55 $ 0.63 $ 0.93 $ 0.86 $ 2.98 Quarter Ended Year Ended Dec. 31, 2017 Mar. 31, June 30, Sept. 30, Dec. 31, (2) Revenues $ 626,528 $ 653,313 $ 667,048 $ 679,035 $ 2,625,924 Gross profit 198,144 212,937 218,315 220,677 850,073 Income from continuing operations 22,382 33,236 34,879 67,377 157,874 Income (loss) from discontinued operations, net of income taxes 9 (139 ) (23 ) (46 ) (199 ) Net income $ 22,391 $ 33,097 $ 34,856 $ 67,331 $ 157,675 Per share income from continuing operations and net income: Basic $ 0.43 $ 0.63 $ 0.66 $ 1.29 $ 3.01 Diluted $ 0.42 $ 0.62 $ 0.66 $ 1.28 $ 2.97 ______ (1) ECS was acquired on April 2, 2018, see Note 3 – Acquisitions . (2) On December 22, 2017, the U.S. government enacted comprehensive tax legislation, the TCJA. In the fourth quarter of 2017, the Company recorded an estimated net tax benefit of $31.4 million for the impact of the TCJA, which is included in the provision for income taxes in the consolidated statement of operations and comprehensive income, see Note 11 – Income Taxes .

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Event [Line Items]
Subsequent Events [Text Block]	Subsequent Event 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.

Schedule II - Valuation and Qua

Schedule II - Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2018
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, 2018 Allowance for doubtful accounts $ 8,449 (1) 3,292 — (6,905 ) $ 4,836 Workers’ compensation loss reserves $ 14,777 3,577 — (972 ) $ 17,382 Year ended December 31, 2017 Allowance for doubtful accounts and billing adjustments $ 8,093 5,251 6,799 (10,234 ) $ 9,909 Workers’ compensation loss reserves $ 15,784 2,908 — (3,915 ) $ 14,777 Year ended December 31, 2016 Allowance for doubtful accounts and billing adjustments $ 6,682 2,881 7,907 (9,377 ) $ 8,093 Workers’ compensation loss reserves $ 15,020 2,693 — (1,929 ) $ 15,784 ______ (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 2018. For the years ended 2017 and 2016 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.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 10 – 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. Office space leases range from six months to 11 years . Rent expense is recognized on a straight-line basis over the lease term. Differences between the straight-line rent expense and amounts payable under the leases are recorded as deferred rent which is included in other current liabilities and other long-term liabilities, as appropriate, in the consolidated balance sheets. This includes the impact of both scheduled rent increases and free or reduced periods. Allowances provided by landlords for leasehold improvements are included in deferred rent, see Note 2 – Accounting Standards Update .
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 st for each of its four reporting units. When evaluating goodwill for impairment, the Company may first perform a qualitative assessment (“step zero” of the impairment test) 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 (“step one” of the impairment test) 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 our 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 2018 , 2017 and 2016 .
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, both per occurrence and in the aggregate. 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 our 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 our ECS segment are primarily from the U.S. government and are considered to have low credit risk.

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block]	The following table summarizes the consider ation paid and the fair value of assets acquired and liabilities assumed (in thousands): Cash $ 12,400 Accounts receivable 97,167 Prepaid expenses and other current assets 8,568 Property and equipment 28,977 Identifiable intangible assets 194,950 Goodwill 528,207 Other non-current assets 1,282 Total assets acquired 871,551 Current liabilities 94,667 Long-term liabilities 4,268 Total liabilities assumed 98,935 Total purchase price $ 772,616
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 thousands): Useful life Contractual customer relationships 12.75 years $ 144,600 Backlog 2.75 years 23,100 Non-compete agreements 4 to 7 years 10,250 Favorable contracts 5 years 500 Trademarks indefinite 16,500 Total identifiable intangible assets acquired $ 194,950 The weighted-average amortization period for identifiable intangible assets, excluding trademarks, is 11 years .
Business Acquisition, Pro Forma Information [Table Text Block]	The summary below (in thousands, 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 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 $ 3,548,820 $ 3,213,465 Income from continuing operations $ 169,604 $ 134,830 Net income $ 169,326 $ 134,632 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 52,353 52,503 Diluted 53,164 53,245

Goodwill and Identifiable Ass_2

Goodwill and Identifiable Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The changes in the carrying amount of goodwill for the years ended December 31, 2018 and 2017 are as follows (in thousands): Apex Segment Oxford Segment ECS Segment Total Balance as of December 31, 2016 $ 644,617 $ 228,896 $ — $ 873,513 Stratacuity acquisition 17,467 — — 17,467 Translation adjustment — 3,115 — 3,115 Balance as of December 31, 2017 662,084 232,011 — 894,095 ECS acquisition — — 528,207 528,207 Translation adjustment — (1,226 ) — (1,226 ) Balance as of December 31, 2018 $ 662,084 $ 230,785 $ 528,207 $ 1,421,076
Schedule of Acquired Intangible Assets	As of December 31, 2018 and 2017 , the Company had the following acquired intangible assets (in thousands): 2018 2017 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 $ 346,799 $ 145,424 $ 201,375 $ 202,588 $ 119,272 $ 83,316 Contractor relationships 2 - 5 years 71,104 67,072 4,032 71,121 59,174 11,947 Backlog 2.75 years 23,100 17,740 5,360 — — — Non-compete agreements 2 - 7 years 22,128 9,907 12,221 11,850 6,600 5,250 In-use software 6 years 18,900 15,966 2,934 18,900 12,816 6,084 Favorable contracts 5 years 1,400 853 547 900 673 227 483,431 256,962 226,469 305,359 198,535 106,824 Not subject to amortization: Trademarks 262,222 — 262,222 245,942 — 245,942 Total $ 745,653 $ 256,962 $ 488,691 $ 551,301 $ 198,535 $ 352,766
Schedule of Estimated Future Amortization Expense	Estimated amortization for each of the next five years and thereafter follows (in thousands): 2019 $ 47,280 2020 36,401 2021 30,705 2022 22,970 2023 19,622 Thereafter 69,491 $ 226,469

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment at December 31, 2018 and 2017 consisted of the following (in thousands): 2018 2017 Computer hardware and software $ 159,357 $ 115,444 Furniture, fixtures and equipment 22,242 17,556 Leasehold improvements 19,229 16,628 Work-in-progress 3,690 4,565 204,518 154,193 Less -- accumulated depreciation (125,395 ) (96,197 ) $ 79,123 $ 57,996

Long-Term Debt (Tables)

Long-Term Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Long-term Debt	At December 31, 2018 and 2017 , long-term debt consisted of the following (in thousands): 2018 2017 $200 million revolving credit facility, due March 31, 2023 $ — $ — Term B loan facility, due June 5, 2022 337,000 588,000 Term B loan facility, due April 2, 2025 787,000 — 1,124,000 588,000 Unamortized deferred loan costs (23,576 ) (12,787 ) $ 1,100,424 $ 575,213

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Contractual Cash Obligation Payments	The following is a summary of the Company's specified contractual cash obligations as of December 31, 2018 , excluding current liabilities that are included in the consolidated balance sheet (in thousands): Operating Leases Related Party Leases Purchase Obligations Total 2019 $ 29,664 $ 1,285 $ 23,162 $ 54,111 2020 24,103 1,317 5,424 30,844 2021 19,762 1,350 3,615 24,727 2022 14,262 1,383 — 15,645 2023 8,859 1,418 — 10,277 Thereafter 6,285 1,082 — 7,367 Total $ 102,935 $ 7,835 $ 32,201 $ 142,971

Stock-based Compensation and _2

Stock-based Compensation and Other Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	A summary of the status of the Company’s unvested RSUs as of December 31, 2018 and changes during the year then ended are presented below: Service Conditions Performance and Service Conditions Total Weighted Average Grant-Date Fair Value Per Unit Unvested RSUs outstanding at December 31, 2017 207,771 652,202 859,973 $ 44.29 Granted 343,912 389,480 733,392 $ 74.61 Vested (110,500 ) (107,941 ) (218,441 ) $ 43.71 Forfeited (16,502 ) (40,475 ) (56,977 ) $ 53.23 Unvested RSUs outstanding at December 31, 2018 424,681 893,266 1,317,947 $ 60.87 Unvested and expected to vest RSUs outstanding at December 31, 2018 361,250 859,007 1,220,257 $ 60.49
Schedule of Other Share-based Compensation, Activity [Table Text Block]	The following table summarizes the balance of liability awards, expense and settlements (for vested awards) during the years presented (in thousands): 2018 2017 2016 Liability awards beginning of year $ — $ 465 $ — Expense for grants 503 470 465 Settled (6 ) (935 ) — Liability awards end of year $ 497 $ — $ 465
Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block]	The following summarizes pricing and term information for options outstanding as of December 31, 2018 : Options Outstanding Options Exercisable Number Outstanding at Weighted Average Remaining Contractual Life (years) Weighted Average Exercise Price Number Exercisable at Weighted Average Exercise Price Range of Exercise Prices December 31, 2018 December 31, 2018 $ 6.89 - $ 7.31 12,936 1.1 $ 7.09 12,936 $ 7.09 $ 7.39 - $ 7.39 3,500 1.3 $ 7.39 3,500 $ 7.39 $ 8.26 - $ 8.26 9,256 2.1 $ 8.26 9,256 $ 8.26 $ 10.46 - $ 10.46 12,170 2.9 $ 10.46 12,170 $ 10.46 $ 16.51 - $ 16.51 50,000 3.7 $ 16.51 50,000 $ 16.51 $ 6.89 - $ 16.51 87,862 2.9 $ 13.05 87,862 $ 13.05
Share-based Compensation, Stock Options, Activity [Table Text Block]	The following table is a summary of stock option activity during 2018 : Non- Qualified Stock Options Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value Outstanding at December 31, 2017 132,670 $ 12.79 3.8 $ 6,830,000 Exercised (44,466 ) $ 12.32 Canceled (342 ) $ 5.18 Outstanding at December 31, 2018 87,862 $ 13.05 2.9 $ 3,642,000 Vested and Expected to Vest at December 31, 2018 87,862 $ 13.05 2.9 $ 3,642,000 Exercisable at December 31, 2018 87,862 $ 13.05 2.9 $ 3,642,000
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 (dollars in thousands, except per share amounts): Year Ended December 31, Average Black-Scholes Valuation per Share Shares Expense 2018 $15.09 160,407 $ 2,740 2017 $9.71 214,466 $ 2,092 2016 $9.05 242,303 $ 2,497

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Components of Income Taxes	The provision for income taxes consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 Current: Federal $ 20,550 $ 33,623 $ 38,103 State 10,617 7,717 6,685 Foreign 3,896 2,725 2,672 35,063 44,065 47,460 Deferred: Federal and State 11,471 (4,726 ) 13,169 Foreign (343 ) (120 ) (426 ) 11,128 (4,846 ) 12,743 $ 46,191 $ 39,219 $ 60,203
Income (Loss) before Income Tax Provision	Income from continuing operations before income taxes consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 United States $ 190,688 $ 184,168 $ 148,402 Foreign 13,487 12,925 8,997 $ 204,175 $ 197,093 $ 157,399
Schedule of Deferred Tax Assets and Liabilities	The components of deferred tax assets (liabilities) are as follows (in thousands): December 31, 2018 2017 Intangibles $ (90,834 ) $ (73,921 ) Depreciation expense (14,983 ) (9,851 ) Allowance for doubtful accounts 1,600 2,615 Employee-related accruals 9,608 2,616 Stock-based compensation 9,506 4,766 Other 5,270 4,339 Net operating loss carryforwards–foreign 999 1,255 Valuation allowance (999 ) (1,255 ) $ (79,833 ) $ (69,436 )
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 2018 and 35 percent in 2017 and 2016 to income before income taxes, for each respective year, and the income tax provision is as follows (in thousands): Year Ended December 31, 2018 2017 2016 Income tax provision at the statutory rate $ 42,877 $ 68,983 $ 55,089 State income taxes, net of federal benefit 9,432 6,751 4,873 Disallowed meals and entertainment expenses 1,591 1,854 1,814 Excess stock-based compensation benefit (2,193 ) (4,241 ) (165 ) Work opportunity tax credit (3,081 ) (1,950 ) (1,463 ) Impact of TCJA (2,992 ) (31,420 ) — Other 557 (758 ) 55 $ 46,191 $ 39,219 $ 60,203
Schedule of Unrecognized Tax Benefits Roll Forward	The following is a reconciliation of the total amounts of unrecognized tax benefits (in thousands): Year Ended December 31, 2018 2017 2016 Unrecognized tax benefit beginning of year $ 430 $ 1,293 $ 814 Gross increases - tax positions in current year 20 71 — Gross increases - tax positions in prior year 69 — 479 Gross decreases - tax positions in prior year — (254 ) — Lapse of the statute of limitations (130 ) (680 ) — Unrecognized tax benefit end of year $ 389 $ 430 $ 1,293

Earnings per Share (Tables)

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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 thousands): Year Ended December 31, 2018 2017 2016 Weighted average number of common shares outstanding used to compute basic earnings per share 52,333 52,503 53,192 Dilutive effect of stock-based awards 728 702 555 Number of shares used to compute diluted earnings per share 53,061 53,205 53,747

Business Segments (Tables)

Business Segments (Tables)	12 Months Ended
	Dec. 31, 2018
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 thousands): Year ended December 31, 2018 2017 2016 Apex: Revenues $ 2,300,242 $ 2,037,154 $ 1,836,488 Gross profit 687,861 606,294 548,421 Operating income 262,297 222,010 195,133 Amortization 26,185 29,361 34,359 Oxford: Revenues $ 606,510 $ 588,770 $ 603,925 Gross profit 248,875 243,779 246,762 Operating income 54,142 53,787 51,294 Amortization 4,179 4,083 5,269 ECS: Revenues $ 493,029 $ — $ — Gross profit 86,915 — — Operating income 15,514 — — Amortization 28,142 — — Corporate: Operating loss (1) $ (71,805 ) $ (51,061 ) $ (56,701 ) Consolidated: Revenues $ 3,399,781 $ 2,625,924 $ 2,440,413 Gross profit 1,023,651 850,073 795,183 Operating income 260,148 224,736 189,726 Amortization 58,506 33,444 39,628 _________ (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 thousands): Year Ended December 31, 2018 2017 2016 Apex: Assignment $ 2,244,470 $ 1,993,256 $ 1,791,566 Permanent placement 55,772 43,898 44,922 $ 2,300,242 $ 2,037,154 $ 1,836,488 Oxford: Assignment $ 516,045 $ 503,058 $ 520,705 Permanent placement 90,465 85,712 83,220 $ 606,510 $ 588,770 $ 603,925 ECS: Firm-fixed-price $ 133,146 $ — $ — Time and Materials 143,434 — — Cost-plus-fixed-fee 216,449 — — $ 493,029 $ — $ — Consolidated $ 3,399,781 $ 2,625,924 $ 2,440,413
Revenue from External Customers by Products and Services [Table Text Block]	The following table presents the ECS segment revenues by customer type (in thousands): Year Ended December 31, 2018 Department of Defense and Intelligence Agencies $ 310,984 Federal Civilian 150,922 Commercial and Other 31,123 $ 493,029
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 thousands): Year Ended December 31, 2018 2017 2016 Domestic $ 3,241,804 $ 2,494,446 $ 2,324,713 Foreign 157,977 131,478 115,700 $ 3,399,781 $ 2,625,924 $ 2,440,413
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	The following table presents total assets by reportable segment (in thousands): December 31, 2018 2017 Apex $ 1,368,548 $ 1,357,246 Oxford 421,199 424,593 ECS 865,610 — Corporate (1) 32,494 28,290 $ 2,687,851 $ 1,810,129
Long-lived Assets by Geographic Areas [Table Text Block]	The following table presents long-lived assets by geographic location (in thousands): December 31, 2018 2017 Domestic $ 76,742 $ 56,137 Foreign 2,381 1,859 $ 79,123 $ 57,996

Unaudited Quarterly Financial_2

Unaudited Quarterly Financial Results (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Information	Quarter Ended Year Ended Dec. 31, 2018 Mar. 31, June 30, (1) Sept. 30, Dec. 31, Revenues $ 685,173 $ 878,509 $ 906,449 $ 929,650 $ 3,399,781 Gross profit 217,737 263,846 270,172 271,896 1,023,651 Income from continuing operations 29,240 33,601 49,217 45,926 157,984 Loss from discontinued operations, net of income taxes (148 ) (40 ) (45 ) (45 ) (278 ) Net income $ 29,092 $ 33,561 $ 49,172 $ 45,881 $ 157,706 Per share income from continuing operations and net income: Basic $ 0.56 $ 0.64 $ 0.94 $ 0.88 $ 3.02 Diluted $ 0.55 $ 0.63 $ 0.93 $ 0.86 $ 2.98 Quarter Ended Year Ended Dec. 31, 2017 Mar. 31, June 30, Sept. 30, Dec. 31, (2) Revenues $ 626,528 $ 653,313 $ 667,048 $ 679,035 $ 2,625,924 Gross profit 198,144 212,937 218,315 220,677 850,073 Income from continuing operations 22,382 33,236 34,879 67,377 157,874 Income (loss) from discontinued operations, net of income taxes 9 (139 ) (23 ) (46 ) (199 ) Net income $ 22,391 $ 33,097 $ 34,856 $ 67,331 $ 157,675 Per share income from continuing operations and net income: Basic $ 0.43 $ 0.63 $ 0.66 $ 1.29 $ 3.01 Diluted $ 0.42 $ 0.62 $ 0.66 $ 1.28 $ 2.97 ______ (1) ECS was acquired on April 2, 2018, see Note 3 – Acquisitions . (2) On December 22, 2017, the U.S. government enacted comprehensive tax legislation, the TCJA. In the fourth quarter of 2017, the Company recorded an estimated net tax benefit of $31.4 million for the impact of the TCJA, which is included in the provision for income taxes in the consolidated statement of operations and comprehensive income, see Note 11 – Income Taxes .

Summary of Significant Accoun_3

Summary of Significant Accounting Policies Lease Term (Details)	Dec. 31, 2018
Minimum [Member]
Lessee, Lease, Description [Line Items]
Lessee Operating Lease Term Of Contract	6 months
Maximum [Member]
Lessee, Lease, Description [Line Items]
Lessee Operating Lease Term Of Contract	11 years

Accounting Standards Update Acc

Accounting Standards Update Accounting Standards Update Leases (Details) $ in Millions	Dec. 31, 2018USD ($)
Leases [Abstract]
Operating Lease, Right-of-Use Asset	$ 95
Operating Lease, Liability	$ 100

Acquisitions Acquisition Costs,

Acquisitions Acquisition Costs, by Acquisition (Details) - USD ($) $ in Thousands	Apr. 02, 2018	Aug. 08, 2017	Dec. 31, 2018	Dec. 31, 2017
Stratacuity
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Aug. 8, 2017
Business Acquisition, Name of Acquired Entity		StratAcuity Staffing Partners, Inc.
Payments to Acquire Businesses, Gross		$ 25,900
Business Acquisition, Description of Acquired Entity		headquartered in New Hampshire
Business Combination, Reason for Business Combination		was purchased to expand the Company's specialized clinical/scientific staffing solutions for biotechnology and pharmaceutical organizations
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual				$ 7,500
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual				$ 600
Business Acquisition, Goodwill, Expected Tax Deductible Amount		$ 17,500
Business Combination, Acquisition Related Costs		500
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		$ 7,600
ECS
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Apr. 2, 2018
Business Acquisition, Name of Acquired Entity	ECS Federal, LLC
Payments to Acquire Businesses, Gross	$ 775,000
Business Acquisition, Transaction Costs			$ 12,040
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
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual			493,029
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual			14,200
Business Acquisition, Goodwill, Expected Tax Deductible Amount			$ 514,240

Acquisitions Schedule of Purcha

Acquisitions Schedule of Purchase Price Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Apr. 02, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 1,421,076		$ 894,095
ECS
Business Acquisition [Line Items]
Cash		$ 12,400
Accounts receivable		97,167
Prepaid expenses and other current assets		8,568
Property and equipment		28,977
Identifiable intangible assets		194,950
Goodwill		528,207
Other non-current assets		1,282
Total assets acquired		871,551
Current liabilities		94,667
Long-term liabilities		4,268
Total liabilities assumed		98,935
Total purchase price		$ 772,616

Acquisitions Schedule of Intang

Acquisitions Schedule of Intangible Assets Acquired as Part of Business Combination (Details) - ECS $ in Thousands	Apr. 02, 2018USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Identifiable intangible assets	$ 194,950
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	$ 16,500
Contractual customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	144,600
Backlog
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	23,100
Non-compete agreements
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	10,250
Favorable contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 500

Acquisitions Business Acquisiti

Acquisitions Business Acquisition, Pro Forma Revenue (Details) - ECS - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues	$ 3,548,820	$ 3,213,465
Income from continuing operations	169,604	134,830
Net income	$ 169,326	$ 134,632
Basic	$ 3.24	$ 2.57
Diluted	$ 3.19	$ 2.53
Basic Shares	52,353	52,503
Diluted Shares	53,164	53,245

Goodwill and Identifiable Ass_3

Goodwill and Identifiable Assets Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Amortization of intangible assets	$ 58,506	$ 33,444	$ 39,628
Goodwill [Roll Forward]
Gross goodwill, period start	894,095	873,513
Goodwill, Acquired During Period	528,207	17,467
Translation adjustment	(1,226)	3,115
Gross goodwill, period end	1,421,076	894,095	873,513
Apex
Goodwill [Line Items]
Amortization of intangible assets	26,185	29,361	34,359
Goodwill [Roll Forward]
Gross goodwill, period start	(662,084)	644,617
Translation adjustment	0	0
Gross goodwill, period end	(662,084)	(662,084)	644,617
Oxford
Goodwill [Line Items]
Amortization of intangible assets	4,179	4,083	5,269
Goodwill [Roll Forward]
Gross goodwill, period start	(232,011)	228,896
Translation adjustment	(1,226)	3,115
Gross goodwill, period end	(230,785)	(232,011)	$ 228,896
ECS
Goodwill [Line Items]
Amortization of intangible assets	28,142
Goodwill [Roll Forward]
Goodwill, Acquired During Period	528,207
Gross goodwill, period end	$ 528,207
Stratacuity | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 17,467

Goodwill and Identifiable Ass_4

Goodwill and Identifiable Assets Acquired Intangible Assets (Details) - USD ($) $ in Thousands	Apr. 02, 2018	Dec. 31, 2018	Dec. 31, 2017
Intangible Assets [Line Items]
Identifiable intangible assets, net		$ 488,691	$ 352,766
Intangible Assets, Gross (Excluding Goodwill)		745,653	551,301
Intangible assets subject to amortization:
Gross Carrying Amount		483,431	305,359
Accumulated Amortization		256,962	198,535
Net Carrying Amount		226,469	106,824
Intangible assets not subject to amortization:
Trademarks, Carrying Amount		262,222	245,942
Contractual customer relationships
Intangible Assets [Line Items]
Estimated Useful Life	12 years 9 months
Intangible assets subject to amortization:
Gross Carrying Amount		346,799	202,588
Accumulated Amortization		145,424	119,272
Net Carrying Amount		$ 201,375	83,316
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		$ 71,104	71,121
Accumulated Amortization		67,072	59,174
Net Carrying Amount		$ 4,032	11,947
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		$ 23,100
Accumulated Amortization		17,740
Net Carrying Amount		$ 5,360
Backlog | Minimum [Member]
Intangible Assets [Line Items]
Estimated Useful Life		2 years 9 months
Non-compete agreements
Intangible assets subject to amortization:
Gross Carrying Amount		$ 22,128	11,850
Accumulated Amortization		9,907	6,600
Net Carrying Amount		$ 12,221	5,250
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	7 years
In-use software
Intangible assets subject to amortization:
Gross Carrying Amount		$ 18,900	18,900
Accumulated Amortization		15,966	12,816
Net Carrying Amount		$ 2,934	6,084
In-use software | Minimum [Member]
Intangible Assets [Line Items]
Estimated Useful Life		6 years
Favorable contracts
Intangible Assets [Line Items]
Estimated Useful Life	5 years
Intangible assets subject to amortization:
Gross Carrying Amount		$ 1,400	900
Accumulated Amortization		853	673
Net Carrying Amount		$ 547	$ 227
Favorable contracts | Minimum [Member]
Intangible Assets [Line Items]
Estimated Useful Life		5 years

Goodwill and Identifiable Ass_5

Goodwill and Identifiable Assets Future Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2,019	$ 47,280
2,020	36,401
2,021	30,705
2,022	22,970
2,023	19,622
Thereafter	69,491
Net Carrying Amount	$ 226,469	$ 106,824

Property and Equipment (Details

Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment	$ 204,518	$ 154,193
Less -- accumulated depreciation	125,395	96,197
Total	79,123	57,996
Computer hardware and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	159,357	115,444
Furniture, fixtures and equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	22,242	17,556
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	19,229	16,628
Work-in-progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 3,690	$ 4,565

Property and Equipment (Narrati

Property and Equipment (Narratives) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Net book value	$ 79,123	$ 57,996
Depreciation	36,500	25,200	$ 22,600
Cost of Sales [Member]
Property, Plant and Equipment [Line Items]
Depreciation	7,700
Software Development
Property, Plant and Equipment [Line Items]
Net book value	41,300	33,300
Software Development Work-in-Progress
Property, Plant and Equipment [Line Items]
Net book value	$ 3,100	$ 3,200

Long-Term Debt (Details)

Long-Term Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)Rate	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Secured Debt	$ 1,124,000	$ 588,000
Unamortized Debt Issuance Expense	(23,576)	(12,787)
Long-term Debt	1,100,424	575,213
Line of Credit Facility, Maximum Borrowing Capacity	$ 300,000
Pro Forma Consolidated Leverage Ratio | Rate	3.25%
FeesPaidInConnectionWithAmendment	$ 22,450
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	15,300
Interest Expense, Debt	6,160
Contractual Quarterly Payments	$ 2,100
Leverage ratio	2.69
$200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0	0
$825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Secured Debt	337,000	588,000
$822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Secured Debt	787,000	$ 0
Face amount of term loan	$ 822,000
Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio | Rate	4.75%
Leverage Ratio in Latest Year of Debt Term | Rate	3.75%
Term Loan Facility | Minimum [Member] | $200 Million Revolving Credit Facility, Due March 2023 [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] | $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings | Rate	1.25%
Term Loan Facility | Maximum [Member] | $200 Million Revolving Credit Facility, Due March 2023 [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	4.52%
Term Loan Facility | Maximum [Member] | Eurodollar [Member] | $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings | Rate	2.25%
Term Loan Facility | Maximum [Member] | Eurodollar [Member] | Long-term Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate borrowings | Rate	2.00%
Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 195,600

Commitments and Contingencies C

Commitments and Contingencies Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Operating Leases, Rent Expense	$ 32,700	$ 26,400	$ 25,600
Deferred Rent Credit, Current	1,300	1,200
Deferred Rent Credit, Noncurrent	5,600	5,000
Costs and Expenses, Related Party	1,269	1,269	$ 1,269
Self Insurance Reserve	2,400	2,100
Workers' compensation receivable	15,011	$ 12,702
Letters of Credit Outstanding, Amount	$ 4,400

Commitments and Contingencies_2

Commitments and Contingencies Contractual Obligations (Details) $ in Thousands	Dec. 31, 2018USD ($)property
Operating Leases
2,019	$ 29,664
2,020	24,103
2,021	19,762
2,022	14,262
2,023	8,859
Thereafter	6,285
Total	102,935
Related Party Leases
2,019	1,285
2,020	1,317
2,021	1,350
2,022	1,383
2,023	1,418
Thereafter	1,082
Total	7,835
Purchase Obligations
2,019	23,162
2,020	5,424
2,021	3,615
2,022	0
2,023	0
Thereafter	0
Total	32,201
Total Contractual Obligations
2,019	54,111
2,020	30,844
2,021	24,727
2,022	15,645
2,023	10,277
Thereafter	7,367
Total	$ 142,971
Apex Systems [Member]
Other Commitments [Line Items]
Number of Leased Properties Owned by Related Parties | property	2

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jun. 10, 2016
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount			$ 150,000
Stock Repurchased and Retired During Period, Value	$ (58,070)	$ (43,091)
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	(1,229,249)	(1,133,553)
Stock Repurchased and Retired During Period, Value	$ (12)	$ (11)
Additional Paid-in Capital [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Value	(13,229)	(11,885)
Retained Earnings [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ (44,829)	$ (31,195)

Stock-based Compensation and _3

Stock-based Compensation and Other Employee Benefit Plans RSU and RSAs (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	148,434
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)	207,771
Granted (in shares)	343,912
Vested (in shares)	(110,500)
Forfeited (in shares)	(16,502)
Unvested RSUs and RSAs outstanding at December 31 (in shares)	424,681	207,771
Unvested and expected to vest RSUs and RSAs outstanding at December 31 (in shares)	361,250
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)	652,202
Granted (in shares)	389,480
Vested (in shares)	(107,941)
Forfeited (in shares)	(40,475)
Unvested RSUs and RSAs outstanding at December 31 (in shares)	893,266	652,202
Unvested and expected to vest RSUs and RSAs outstanding at December 31 (in shares)	859,007
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	927,572
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)	859,973
Granted (in shares)	733,392
Vested (in shares)	(218,441)
Forfeited (in shares)	(56,977)
Unvested RSUs and RSAs outstanding at December 31 (in shares)	1,317,947	859,973
Unvested and expected to vest RSUs and RSAs outstanding at December 31 (in shares)	1,220,257
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)	$ 44.29
Granted, weighted average grant date fair value (in dollars per share)	74.61	$ 49.62	$ 36.07
Vested, weighted average grant date fair value (in dollars per share)	$ 43.71
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Forfeitures	53.23
Unvested RSUs and RSAs outstanding at December 31, weighted average grant date fair value (in dollars per share)	$ 60.87	$ 44.29
Unvested and expected to vest RSUs and RSAs outstanding at December 31, weighted average grant date fair value (in dollars per share)	$ 60.49
Liability Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Share-based Arrangements, Liability, Current and Noncurrent	$ 497	$ 0	$ 465	$ 0

Stock-based Compensation_ Incen

Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	148,434
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 2,700	$ 4,500	$ 2,700
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	927,572
Shares surrendered by the employees to the Company for payment of minimum tax withholding obligations	63,412
Total intrinsic value of options exercised	$ 16,400	37,800	21,700
Unrecognized compensation expense	$ 41,100
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	$ 497	0	465	$ 0
Liability Awards Granted	503	470	465
Liability Awards Settled During Period	$ 6	$ (935)	$ 0
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, Number of Shares Available for Grant	1,925,448

Stock-based Compensation and _4

Stock-based Compensation and Other Employee Benefit Plans Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan (Range of Prices) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, low end of the range (in dollars per share)	$ 6.89
Exercise price, high end of the range (in dollars per share)	$ 16.51
Options outstanding (in shares)	87,862
Options outstanding - weighted average remaining contractual life (years)	2 years 10 months 24 days	3 years 9 months 18 days
Options outstanding - weighted average exercise price (in dollars per share)	$ 13.05	$ 12.79
Options exercisable - at the end of period (in shares)	87,862
Options exercisable - weighted average exercise price (in dollars per share)	$ 13.05
$6.89 - 7.31
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, low end of the range (in dollars per share)	6.89
Exercise price, high end of the range (in dollars per share)	$ 7.31
Options outstanding (in shares)	12,936
Options outstanding - weighted average remaining contractual life (years)	1 year 1 month 6 days
Options outstanding - weighted average exercise price (in dollars per share)	$ 7.09
Options exercisable - at the end of period (in shares)	12,936
Options exercisable - weighted average exercise price (in dollars per share)	$ 7.09
$7.39 - 7.39
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, low end of the range (in dollars per share)	7.39
Exercise price, high end of the range (in dollars per share)	$ 7.39
Options outstanding (in shares)	3,500
Options outstanding - weighted average remaining contractual life (years)	1 year 3 months 18 days
Options outstanding - weighted average exercise price (in dollars per share)	$ 7.39
Options exercisable - at the end of period (in shares)	3,500
Options exercisable - weighted average exercise price (in dollars per share)	$ 7.39
$8.26 - 8.26
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, low end of the range (in dollars per share)	8.26
Exercise price, high end of the range (in dollars per share)	$ 8.26
Options outstanding (in shares)	9,256
Options outstanding - weighted average remaining contractual life (years)	2 years 1 month 6 days
Options outstanding - weighted average exercise price (in dollars per share)	$ 8.26
Options exercisable - at the end of period (in shares)	9,256
Options exercisable - weighted average exercise price (in dollars per share)	$ 8.26
$10.46 - 10.46
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, low end of the range (in dollars per share)	10.46
Exercise price, high end of the range (in dollars per share)	$ 10.46
Options outstanding (in shares)	12,170
Options outstanding - weighted average remaining contractual life (years)	2 years 10 months 24 days
Options outstanding - weighted average exercise price (in dollars per share)	$ 10.46
Options exercisable - at the end of period (in shares)	12,170
Options exercisable - weighted average exercise price (in dollars per share)	$ 10.46
$16.51 - 16.51
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, low end of the range (in dollars per share)	16.51
Exercise price, high end of the range (in dollars per share)	$ 16.51
Options outstanding (in shares)	50,000
Options outstanding - weighted average remaining contractual life (years)	3 years 8 months 12 days
Options outstanding - weighted average exercise price (in dollars per share)	$ 16.51
Options exercisable - at the end of period (in shares)	50,000
Options exercisable - weighted average exercise price (in dollars per share)	$ 16.51

Stock-based Compensation and _5

Stock-based Compensation and Other Employee Benefit Plans Stock Options Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Outstanding Stock Options Activity [Roll Forward]
Outstanding at December 31 (in shares)	87,862
Exercisable at December 31 (in shares)	87,862
Options Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding - weighted average exercise price per share	$ 12.79
Exercised, weighted average exercise price per share	12.32
Canceled, weighted average exercise price per share	5.18
Vested and Expected to Vest, weighted average exercise price per share	13.05
Exercisable, weighted average exercise price per share	$ 13.05
Outstanding, weighted average remaining contractual term	2 years 10 months 24 days	3 years 9 months 18 days
Vested and Expected to Vest, weighted average remaining contractual term	2 years 10 months 24 days
Exercisable, weighted average remaining contractual term	2 years 10 months 24 days
Options outstanding - aggregate intrinsic value	$ 3,642,000	$ 6,830,000
Vested and Expected to Vest, aggregate intrinsic value	3,642,000
Exercisable, aggregate intrinsic value	3,642,000
Employee Stock Option [Member]
Options Outstanding, Weighted Average Exercise Price [Roll Forward]
Total intrinsic value of options exercised	$ 3,200,000	$ 1,500,000	$ 4,700,000
Non Qualified Stock Options [Member]
Outstanding Stock Options Activity [Roll Forward]
Outstanding at January 1 (in shares)	132,670
Exercised (in shares)	(44,466)
Canceled (in shares)	(342)
Outstanding at December 31 (in shares)	87,862	132,670
Vested and Expected to Vest at December 31(in shares)	87,862
Exercisable at December 31 (in shares)	87,862

Stock-based Compensation and _6

Stock-based Compensation and Other Employee Benefit Plans Employee Stock Purchase Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	148,434
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, Number of Shares Available for Grant	1,925,448
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 15.09	$ 9.71	$ 9.05
Stock Issued During Period, Shares, Employee Stock Purchase Plans	160,407	214,466	242,303
Allocated Share-based Compensation Expense	$ 2,740	$ 2,092	$ 2,497

Stock-based Compensation and _7

Stock-based Compensation and Other Employee Benefit Plans Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Deferred Compensation Plan Assets	$ 6.2
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 13	$ 7.5	$ 5.6

Revenues (Details)

Revenues (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract with Customer, Refund Liability		$ 1,500
Deferred Revenue	$ 9,750	570
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	11,580
Contract with Customer, Liability, Revenue Recognized	11,500
Allowance for Doubtful Accounts Receivable, Current	$ 4,840	9,910
permanent placement fallouts and other revenue adjustments		$ 3,100

Income Taxes (Narratives) (Deta

Income Taxes (Narratives) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
U.S. federal statutory tax rate	21.00%	35.00%	35.00%
TaxBenefitFromRemeasurementofDeferredTaxesDuetoTaxReform	$ 3,000	$ 31,400
Operating Loss Carryforwards, Valuation Allowance	1,000	1,200
Accumulated net earning	586,125	428,419
Gross deferred tax assets	32,500	18,600
Gross deferred tax liabilities	111,300	86,800
Unrecognized Tax Benefits	389	$ 430	$ 1,293	$ 814
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	0
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	0
Foreign Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	1,000
Accumulated net earning	$ 15,500

Income Taxes (Income Tax Compon

Income Taxes (Income Tax Components) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ 20,550	$ 33,623	$ 38,103
State	10,617	7,717	6,685
Foreign	3,896	2,725	2,672
Total Current	35,063	44,065	47,460
Deferred:
Federal & State	11,471	(4,726)	13,169
Foreign	(343)	(120)	(426)
Total Deferred	11,128	(4,846)	12,743
Total	$ 46,191	$ 39,219	$ 60,203

Income Taxes (Income Before Tax

Income Taxes (Income Before Tax) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States	$ 190,688	$ 184,168	$ 148,402
Foreign	13,487	12,925	8,997
Income before income taxes	$ 204,175	$ 197,093	$ 157,399

Income Taxes (Deferred Tax Asse

Income Taxes (Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Intangibles	$ (90,834)	$ (73,921)
Depreciation expense	(14,983)	(9,851)
Allowance for doubtful accounts	1,600	2,615
Employee related accruals	9,608	2,616
Stock-based compensation	9,506	4,766
Other	5,270	4,339
Net operating loss carryforwards - foreign	999	1,255
Valuation allowance	(999)	(1,255)
Total deferred income tax assets (liabilities)	$ (79,833)	$ (69,436)

Income Taxes (Tax Rate Reconcil

Income Taxes (Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax provision at the statutory rate	$ 42,877	$ 68,983	$ 55,089
State income taxes, net of federal benefit	9,432	6,751	4,873
Permanent difference – non deductible items	1,591	1,854	1,814
Proceeds and Excess Tax Benefit from Share-based Compensation	(2,193)	(4,241)	(165)
Work opportunity tax credit	(3,081)	(1,950)	(1,463)
Tax Adjustments, Settlements, and Unusual Provisions	(2,992)	(31,420)	0
Other	557	(758)	55
Total	$ 46,191	$ 39,219	$ 60,203

Income Taxes (Unrecognized Tax

Income Taxes (Unrecognized Tax Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefit beginning of year	$ 430	$ 1,293	$ 814
Gross increases- tax positions in current year	20	71	0
Gross increases- tax positions in prior year	69	0	479
Gross decreases- tax positions in prior year	0	(254)	0
Reduction resulting from lapse of applicable statute of limitations	(130)	(680)	0
Unrecognized Tax Benefit end of year	$ 389	$ 430	$ 1,293

Earnings per Share (Details)

Earnings per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted Average Number of Shares Outstanding Reconciliation
Weighted average number of common shares outstanding used to compute basic earnings per share	52,333	52,503	53,192
Dilutive effect of stock-based awards	728	702	555
Number of shares used to compute diluted earnings per share	53,061	53,205	53,747

Business Segments Segment Repor

Business Segments Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Gross profit	$ 271,896	$ 270,172	$ 263,846	$ 217,737	$ 220,677	$ 218,315	$ 212,937	$ 198,144	1,023,651	850,073	795,183
Operating Income (Loss)									260,148	224,736	189,726
Amortization of intangible assets									58,506	33,444	39,628
Apex
Segment Reporting Information [Line Items]
Revenues									2,300,242	2,037,154	1,836,488
Gross profit									687,861	606,294	548,421
Operating Income (Loss)									262,297	222,010	195,133
Amortization of intangible assets									26,185	29,361	34,359
Oxford
Segment Reporting Information [Line Items]
Revenues									606,510	588,770	603,925
Gross profit									248,875	243,779	246,762
Operating Income (Loss)									54,142	53,787	51,294
Amortization of intangible assets									4,179	4,083	5,269
ECS
Segment Reporting Information [Line Items]
Revenues									493,029	0	0
Gross profit									86,915
Operating Income (Loss)									15,514
Amortization of intangible assets									28,142
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual									493,029
Corporate
Segment Reporting Information [Line Items]
Operating Income (Loss)									$ (71,805)	$ (51,061)	$ (56,701)

Business Segments Disaggregated

Business Segments Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Apex
Revenue from External Customer [Line Items]
Revenues									2,300,242	2,037,154	1,836,488
Apex | Assignment [Member]
Revenue from External Customer [Line Items]
Revenues									2,244,470	1,993,256	1,791,566
Apex | PermanentPlacement [Member]
Revenue from External Customer [Line Items]
Revenues									55,772	43,898	44,922
Oxford
Revenue from External Customer [Line Items]
Revenues									606,510	588,770	603,925
Oxford | Assignment [Member]
Revenue from External Customer [Line Items]
Revenues									516,045	503,058	520,705
Oxford | PermanentPlacement [Member]
Revenue from External Customer [Line Items]
Revenues									90,465	85,712	83,220
ECS
Revenue from External Customer [Line Items]
Revenues									493,029	0	0
ECS | Fixed-price Contract [Member]
Revenue from External Customer [Line Items]
Revenues									133,146	0	0
ECS | Time-and-materials Contract [Member]
Revenue from External Customer [Line Items]
Revenues									143,434	0	0
ECS | Cost-plus-fixed-fee Contract [Member]
Revenue from External Customer [Line Items]
Revenues									$ 216,449	$ 0	$ 0

Business Segments ECS Segment R

Business Segments ECS Segment Revenues by Customer Type (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
department of defense and intelligence agencies [Member]
Revenue from External Customer [Line Items]
Revenues									310,984
federal civilian [Member]
Revenue from External Customer [Line Items]
Revenues									150,922
Commerical and Other [Member]
Revenue from External Customer [Line Items]
Revenues									31,123
ECS
Revenue from External Customer [Line Items]
Revenues									$ 493,029	$ 0	$ 0

Business Segments Revenues by G

Business Segments Revenues by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Domestic [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									3,241,804	2,494,446	2,324,713
Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 157,977	$ 131,478	$ 115,700

Business Segments Total Assets

Business Segments Total Assets by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 2,687,851	$ 1,810,129
Apex
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,368,548	1,357,246
Oxford
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	421,199	424,593
ECS
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	865,610	0
Corporate
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 32,494	$ 28,290

Business Segments Long-lived as

Business Segments Long-lived assets by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Long-Lived Assets	$ 79,123	$ 57,996
Domestic [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Long-Lived Assets	76,742	56,137
Foreign [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Long-Lived Assets	$ 2,381	$ 1,859

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Inputs, Liabilities, Quantitative Information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Secured Debt	$ 1,124,000	$ 588,000
Unamortized Debt Issuance Expense	23,576	$ 12,787
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	1,093,100
Deferred Compensation Plan Assets	$ 6,230

Unaudited Quarterly Financial_3

Unaudited Quarterly Financial Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035		$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Gross profit	271,896	270,172	263,846	217,737	220,677		218,315	212,937	198,144	1,023,651	850,073	795,183
Income from continuing operations	45,926	49,217	33,601	29,240	67,377	[1]	34,879	33,236	22,382	157,984	157,874	97,196
Income from discontinued operations, net of income taxes										(278)	(199)	5
Net income	$ 45,881	$ 49,172	$ 33,561	$ 29,092	$ 67,331		$ 34,856	$ 33,097	$ 22,391	$ 157,706	$ 157,675	$ 97,201
Earnings per share:
Basic	$ 0.88	$ 0.94	$ 0.64	$ 0.56	$ 1.29		$ 0.66	$ 0.63	$ 0.43	$ 3.02	$ 3.01	$ 1.83
Diluted	$ 0.86	$ 0.93	$ 0.63	$ 0.55	$ 1.28		$ 0.66	$ 0.62	$ 0.42	$ 2.98	$ 2.97	$ 1.81
Discontinued Operations, Disposed of by Sale [Member]
Income from discontinued operations, net of income taxes	$ (45)	$ (45)	$ (40)	$ (148)	$ (46)		$ (23)	$ (139)	$ 9	$ (278)	$ (199)
[1]	On December 22, 2017, the U.S. government enacted comprehensive tax legislation, the TCJA. In the fourth quarter of 2017, the Company recorded an estimated net tax benefit of $31.4 million for the impact of the TCJA, which is included in the provision for income taxes in the consolidated statement of operations and comprehensive income, see Note 11 – Income Taxes.

Subsequent Events (Details)

Subsequent Events (Details) - DHA [Member] - Subsequent Event [Member] $ in Millions	Jan. 25, 2019USD ($)
Subsequent Event [Line Items]
Business Acquisition, Effective Date of Acquisition	Jan. 25, 2019
Business Acquisition, Name of Acquired Entity	DHA Group, Inc.
Business Acquisition, Description of Acquired Entity	headquartered in Washington, D.C.
Payments to Acquire Businesses, Gross	$ 46
Business Combination, Reason for Business Combination	DHA was acquired to expand the Company's government services

Schedule II - Valuation and Q_2

Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands		Jan. 01, 2018		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts and Bilinig Adjustments
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year		$ 9,909		$ 9,909	$ 8,093	$ 6,682
Provisions		8,449	[1]	3,292	5,251	2,881
SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account	[2]			0	6,799	7,907
Deductions	[3]			(6,905)	(10,234)	(9,377)
Balance at end of year				4,836	9,909	8,093
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		$ 14,777		14,777	15,784	15,020
Provisions				3,577	2,908	2,693
SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account	[2]			0	0	0
Deductions	[3]			(972)	(3,915)	(1,929)
Balance at end of year				$ 17,382	$ 14,777	$ 15,784
[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 2018. For the years ended 2017 and 2016 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.