Goodwill and Identifiable Assets Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Roll Forward]
Gross goodwill, period start	$ 1,891.9	$ 1,569.5
Purchase price adjustment	1.1	(8.1)
Translation adjustment	1.1	(1.4)
Gross goodwill, period end	1,894.1	1,891.9
Commercial Business
Goodwill [Roll Forward]
Gross goodwill, period start	1,074.7	829.3
Purchase price adjustment	0.0	0.4
Translation adjustment	1.1	(1.4)
Gross goodwill, period end	1,075.8	1,074.7
Federal Government Business
Goodwill [Roll Forward]
Gross goodwill, period start	817.2	740.2
Purchase price adjustment	1.1	(8.5)
Translation adjustment	0.0	0.0
Gross goodwill, period end	818.3	817.2
2022 Acquisitions
Goodwill [Roll Forward]
Acquisitions		331.9
Goodwill, expected tax deductible amount	$ 250.7
2022 Acquisitions | Commercial Business
Goodwill [Roll Forward]
Acquisitions		246.4
2022 Acquisitions | Federal Government Business
Goodwill [Roll Forward]
Acquisitions		$ 85.5