Property, Equipment, and Software - Schedule of Property, Equipment and Software (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	$ 108.4	$ 102.9
Less accumulated depreciation and amortization	(59.4)	(54.6)
Property, equipment and software, net	49.0	48.3
Capitalized computer software additions	1.4
Computer and other equipment
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	31.1	28.7
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	22.2	22.3
Software
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	47.7	44.5
Office furniture
Property, Plant and Equipment [Line Items]
Property, equipment and software, gross	7.4	$ 7.4
Other accrued expenses
Property, Plant and Equipment [Line Items]
Capital lease obligations	0.7
Other non-current liabilities
Property, Plant and Equipment [Line Items]
Capital lease obligations	$ 0.7