Derivatives (Risk Ratings of the Notional Amount of Risk Participation Agreements) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value
Notional amount of the risk participation agreements	$ 2,820	$ 2,467
Pass
Derivatives, Fair Value
Notional amount of the risk participation agreements	2,744	2,447
Risk Level, Special Mention
Derivatives, Fair Value
Notional amount of the risk participation agreements	66	14
Risk Level, Substandard
Derivatives, Fair Value
Notional amount of the risk participation agreements	$ 10	$ 6