Property, Equipment, and Software - Schedule of Property, Equipment and Software (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	$ 269.7	$ 224.9
Less accumulated depreciation and amortization	(170.4)	(131.2)
Property, equipment and software, net	99.3	93.7
Buildings and land
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	3.8	4.6
Computer and other equipment
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	59.0	55.0
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	23.3	23.2
Software
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	177.1	135.7
Office furniture
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	$ 6.5	$ 6.4