Segment Reporting Revenue by Customer Type (details) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer [Line Items]
Revenues	$ 1,011.9	$ 1,002.7	$ 2,939.2	$ 2,898.7
Department of Defense and Intelligence Agencies
Revenue from External Customer [Line Items]
Revenues	172.0	112.6	414.4	316.2
Federal Civilian
Revenue from External Customer [Line Items]
Revenues	96.2	82.0	276.1	213.5
Other
Revenue from External Customer [Line Items]
Revenues	20.4	11.5	50.5	35.0
ECS
Revenue from External Customer [Line Items]
Revenues	$ 288.6	$ 206.1	$ 741.0	$ 564.7