Gains and Losses Recorded in Consolidated Statement of Income Related to Fair Value Hedges (Detail) - Asset And Liability Management - Fair Value Hedges - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$ 68.4	$ 13.6	$ 67.6
Securities Available for Sale | Interest Rate Swaps | Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	63.4	(21.1)	(36.4)
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	$ 5.0	$ 34.7	$ 104.0