Derivative Instruments- Ineffectiveness Related to Derivatives and Hedging Relationships Recorded in Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness related to derivatives and hedging relationships	$ (2.3)		$ (15.5)		$ (7.7)
Fair value hedges of securities
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness related to derivatives and hedging relationships	1.4		(12.9)		(4.9)
Fair value hedges of deposits and long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness related to derivatives and hedging relationships	(3.7)		(2.6)		(2.8)
Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness related to derivatives and hedging relationships	0		0		0.1
Other
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness related to derivatives and hedging relationships	$ 0	[1]	$ 0	[1]	$ (0.1)	[1]

[1] Includes ineffectiveness recorded on foreign exchange hedges.