Segment Reporting - Federal Government Segment Revenues by Customer Type (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenue from External Customer [Line Items]
Revenues	$ 1,053.8	$ 1,073.8	$ 974.9	$ 907.0	$ 900.4	$ 904.4	$ 831.9	$ 865.4	$ 4,009.5	$ 3,502.1	$ 3,415.6
Federal Government
Revenue from External Customer [Line Items]
Revenues									1,082.4	1,004.2	798.2
Federal Government | Department of Defense and Intelligence Agencies
Revenue from External Customer [Line Items]
Revenues									589.7	558.5	453.9
Federal Government | Federal Civilian
Revenue from External Customer [Line Items]
Revenues									421.8	370.6	293.6
Federal Government | Other
Revenue from External Customer [Line Items]
Revenues									$ 70.9	$ 75.1	$ 50.7