Income Taxes - Deferred Tax Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Liabilities:
Lease Financing	$ 85.8	$ 148.7	$ 272.6
Software Development	187.8	352.0	339.9
Accumulated Depreciation	41.0	26.0	20.6
Compensation and Benefits	0.0	50.2	70.7
State Taxes, net	59.4	33.3	48.8
Other Liabilities	145.7	243.1	169.1
Gross Deferred Tax Liabilities	519.7	853.3	921.7
Deferred Tax Assets:
Allowance for Credit Losses	32.3	67.2	81.7
Compensation and Benefits	35.5	0.0	0.0
Other Assets	88.3	233.8	185.0
Gross Deferred Tax Assets	156.1	301.0	266.7
Valuation Reserve	(1.1)	(0.9)	(1.6)
Deferred Tax Assets, net of Valuation Reserve	155.0	300.1	265.1
Net Deferred Tax Liabilities	$ 364.7	$ 553.2	$ 656.6