Acquisitions Business Combinations Additional Information (Details) - USD ($) $ in Millions			3 Months Ended
	Jan. 25, 2019	Apr. 02, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired			$ 48.8	$ 0.0
Goodwill			$ 1,445.5		$ 1,421.1
DHA
Business Acquisition [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.
Working capital excess of target working capital	$ 2.8
ECS
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Apr. 02, 2018
Business Acquisition, Name of Acquired Entity		ECS Federal, LLC
Business Acquisition, Description of Acquired Entity		headquartered in Fairfax, Virginia
Payments to Acquire Businesses, Gross		$ 775.0
Business Combination, Reason for Business Combination		The ECS acquisition allows the Company to compete in the Federal IT and professional services sector.
Goodwill		$ 528.2
Business Acquisition, Goodwill, Expected Tax Deductible Amount		514.2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets		$ 195.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		11 years