Derivatives (Derivative Financial Instruments - Additional Information) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivative
Valuation adjustments related to the credit risk associated with counterparties of customer accomodation derivative contracts	$ 2	$ 3
Notional amount of the risk participation agreements	$ 3,370	2,838
Interest Rate Contract | Credit Risk
Derivative
Credit Risk Derivatives Average Remaining Life	4 years 2 months 12 days
Fair value of risk participation agreements	$ 6	5
Interest Rate Contract | Cash Flow Hedging
Derivative
Maximum length of time hedged in cash flow hedge	4 years 11 months
Deferred gains or losses, net of tax, on cash flow hedges were recorded in accumulated other comprehensive income	$ 19	9
Net deferred gains or losses, net of tax, recorded in accumulated other comprehensive income are expected to be reclassified into earnings during the next twelve months	2
Total collateral
Derivative
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	375	409
Derivative Liability, Fair Value, Amount Offset Against Collateral	322	$ 365
Amount of variation margin payment applied to derivative asset contracts	85
Amount of variation margin payment applied to derivative liability contracts	$ 100