Business Segments ECS Segment Revenues by Customer Type (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	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
Revenue from External Customer [Line Items]
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
ECS
Revenue from External Customer [Line Items]
Revenues										798.2	493.0	$ 0.0
ECS | department of defense and intelligence agencies [Member]
Revenue from External Customer [Line Items]
Revenues										453.9	311.0
ECS | federal civilian [Member]
Revenue from External Customer [Line Items]
Revenues										293.6	150.9
ECS | Commerical and Other [Member]
Revenue from External Customer [Line Items]
Revenues										$ 50.7	$ 31.1

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