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	Mar. 31, 2015	Dec. 31, 2014
Securities available-for-sale
Derivative [Line Items]
Carrying amount	$ 7,233	$ 7,294
Notional amount of hedge	6,966	7,045
Unrealized Gain	4	4
Unrealized (Loss)	(539)	(370)
Long-term debt
Derivative [Line Items]
Carrying amount	16,623	16,469
Notional amount of hedge	16,100	16,100
Unrealized Gain	568	470
Unrealized (Loss)	$ (14)	$ (14)