Segment Reporting Revenue by Customer Type (details) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue from External Customer [Line Items]
Revenues	$ 1,002.7	$ 906.4	$ 2,898.7	$ 2,470.1
Department of Defense and Intelligence Agencies
Revenue from External Customer [Line Items]
Revenues	112.6	101.5	316.2	197.0
Federal Civilian
Revenue from External Customer [Line Items]
Revenues	82.0	52.6	213.5	103.6
Other
Revenue from External Customer [Line Items]
Revenues	11.5	9.9	35.0	18.5
ECS
Revenue from External Customer [Line Items]
Revenues	$ 206.1	$ 164.0	$ 564.7	$ 319.1