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	Mar. 31, 2016	Dec. 31, 2015
Securities available-for-sale
Derivative [Line Items]
Carrying amount	$ 8,602	$ 7,978
Notional amount of hedge	8,025	7,918
Unrealized Gain	0	16
Unrealized (Loss)	(796)	(359)
Long-term debt
Derivative [Line Items]
Carrying amount	18,667	18,231
Notional amount of hedge	17,950	17,850
Unrealized Gain	754	479
Unrealized (Loss)	$ (22)	$ (14)