Segment Reporting Revenue by Customer Type (details) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue from External Customer [Line Items]
Revenues	$ 972.3	$ 878.5	$ 1,896.0	$ 1,563.7
Department of Defense and Intelligence Agencies
Revenue from External Customer [Line Items]
Revenues	107.6	95.5	203.6	95.5
Federal Civilian
Revenue from External Customer [Line Items]
Revenues	71.3	51.0	131.5	51.0
Commercial and Other
Revenue from External Customer [Line Items]
Revenues	11.7	8.6	23.5	8.6
ECS
Revenue from External Customer [Line Items]
Revenues	$ 190.6	$ 155.1	$ 358.6	$ 155.1