Derivatives and Hedging Pre-Tax Net Gains (Losses) on Fair Value Hedges (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	$ 15.2	$ (10.8)	$ 48.7	$ 27.9
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	(15.2)	10.8	(48.7)	(27.9)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	0.0	3.7	0.0	(1.9)
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	$ 0.0	$ (3.7)	$ 0.0	$ 1.9