Derivatives (Risk Ratings of the Notional Amount of Risk Participation Agreements) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value
Notional amount of the risk participation agreements	$ 3,370	$ 2,838
Pass
Derivatives, Fair Value
Notional amount of the risk participation agreements	3,263	2,748
Risk Level, Special Mention
Derivatives, Fair Value
Notional amount of the risk participation agreements	77	66
Risk Level, Substandard
Derivatives, Fair Value
Notional amount of the risk participation agreements	$ 30	$ 24