Derivatives and Hedging Pre-Tax Net Gains (Losses) on Fair Value Hedges (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	$ (45.6)	$ 32.2	$ 30.3
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	44.8	(32.2)	(30.3)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	2.7	(3.1)	(19.3)
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	$ (2.7)	$ 3.1	$ 19.3