Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	$ 1,664	$ 1,113
Pass
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	1,650	1,052
Risk Level, Special Mention
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	7	59
Risk Level, Substandard
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	$ 7	$ 2