Acquisitions Acquisition Costs, by Acquisition (Details) - USD ($) $ in Millions				3 Months Ended									12 Months Ended
	Oct. 17, 2019	Jan. 25, 2019	Apr. 02, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	[1]	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill				$ 1,486.9				$ 1,421.1					$ 1,486.9	$ 1,421.1
Goodwill, Acquired During Period														528.2
Revenues				$ 1,025.2	$ 1,002.7	$ 972.3	$ 923.7	$ 929.7	$ 906.4	$ 878.5		$ 685.2	3,923.9	3,399.8	$ 2,626.0
Intersys
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Oct. 17, 2019
Business Acquisition, Name of Acquired Entity	Intersys Consulting, LLC
Payments to Acquire Businesses, Gross	$ 67.0
Business Acquisition, Description of Acquired Entity	headquartered in Austin, Texas
Business Combination, Reason for Business Combination	The acquisition expands the Company's capabilities in digital innovation and enterprise solutions
Goodwill, Acquired During Period	$ 41.4												41.4
Business Acquisition, Goodwill, Expected Tax Deductible Amount	38.7
Identifiable intangible assets	$ 23.8
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	9 years
DHA
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Jan. 25, 2019
Business Acquisition, Name of Acquired Entity		DHA Group, Inc
Payments to Acquire Businesses, Gross		$ 46.0
Business Acquisition, Description of Acquired Entity		headquartered in Washington, D.C.
Business Combination, Reason for Business Combination		DHA was acquired to expand the Company's government services
Goodwill, Acquired During Period		$ 24.7											24.7
Identifiable intangible assets		$ 19.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		9 years
Apex
Business Acquisition [Line Items]
Revenues													2,520.0	2,300.3	2,037.2
Apex | Intersys
Business Acquisition [Line Items]
Goodwill, Acquired During Period													41.4
ECS
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition			Apr. 02, 2018
Business Acquisition, Name of Acquired Entity			ECS Federal, LLC
Payments to Acquire Businesses, Gross			$ 775.0
Business Acquisition, Transaction Costs			$ 12.0
Business Acquisition, Description of Acquired Entity			headquartered in Fairfax, Virginia
Business Combination, Reason for Business Combination			The ECS acquisition allows the Company to compete in the federal IT and professional services sector
Goodwill			$ 528.2
Revenues													798.2	493.0	0.0
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual													798.2	493.0	$ 0.0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual														$ 14.2
Business Acquisition, Goodwill, Expected Tax Deductible Amount			514.2
Identifiable intangible assets			$ 195.0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			11 years
ECS | DHA
Business Acquisition [Line Items]
Goodwill, Acquired During Period													$ 24.7

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