Fair Value Measurement- Summary of Carrying Amount of Hedged Financial Instruments, Related Notional Amount of Hedge and Estimated Fair Value of Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Interest-bearing deposits with banks
Derivative [Line Items]
Carrying amount	$ 292	$ 1,396
Notional amount of hedge	292	1,396
Unrealized Gain	20	30
Unrealized (Loss)	0	(19)
Securities available-for-sale
Derivative [Line Items]
Carrying amount	6,772	5,914
Notional amount of hedge	6,971	6,647
Unrealized Gain	227	721
Unrealized (Loss)	(129)	(95)
Long-term debt
Derivative [Line Items]
Carrying amount	17,534	15,036
Notional amount of hedge	17,300	14,755
Unrealized Gain	344	483
Unrealized (Loss)	$ (49)	$ (72)