Derivative Financial Instruments - Location and Amount of Fair Value Hedge Derivative Gains and Losses Recorded in Income (Detail) - Asset And Liability Management - Fair Value Hedges - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$ 2.6	$ 68.4	$ 13.6
Securities Available for Sale | Interest Rate Swaps | Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	(0.8)	63.4	(21.1)
Senior Notes and Long- Term Subordinated Debt | Interest Rate Swaps | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$ 3.4	$ 5.0	$ 34.7