Acquisitions Acquisition Costs, by Acquisition (Details) - USD ($) $ in Thousands			12 Months Ended
	Apr. 02, 2018	Aug. 08, 2017	Dec. 31, 2018	Dec. 31, 2017
Stratacuity
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Aug. 08, 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. 02, 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