Segment Reporting - Segment Reporting Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of Reportable Segments | segment										3
Segment Reporting Information [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
Gross profit	$ 313.7	$ 308.7	$ 276.3	$ 243.7	$ 240.8	$ 236.3	$ 227.5	$ 242.6	1,142.4	947.2	971.5
Operating Income (Loss)									350.9	281.2	276.2
Depreciation									28.0	28.3	28.0
Amortization									55.7	51.0	50.3
Commercial Business
Segment Reporting Information [Line Items]
Revenues									2,927.1	2,497.9	2,617.4
Gross profit									934.9	778.3	830.4
Operating Income (Loss)									355.9	284.5	297.8
Depreciation									13.9	14.4	14.6
Amortization									25.7	22.9	23.2
Federal Government Business
Segment Reporting Information [Line Items]
Revenues									1,082.4	1,004.2	798.2
Gross profit									207.6	168.9	141.1
Operating Income (Loss)									76.1	58.0	43.4
Depreciation									8.5	9.0	13.3
Amortization									$ 30.0	$ 28.1	$ 27.1