Acquisitions Business Combinations Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
	Apr. 02, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Business Combinations [Abstract]
Business Acquisition, Effective Date of Acquisition	Apr. 02, 2018
Business Acquisition, Name of Acquired Entity	ECS, headquartered in Fairfax, Virginia
Payments to Acquire Businesses, Gross			$ 775,000
Business Acquisition, Transaction Costs		$ 12,040	12,040
Business Combination, Reason for Business Combination	was purchased to complement and elevate our offerings and strengthen the Company's position as a premier IT and professional services provider by entering the government services space
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual		164,000	319,100
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual		4,310	8,010
Goodwill	$ 527,388	1,420,636	1,420,636	$ 894,095
Business Acquisition, Goodwill, Expected Tax Deductible Amount		$ 507,600	$ 507,600