Derivatives (Derivative Financial Instruments - Additional Information) (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative
Valuation adjustments related to the credit risk associated with counterparties of customer accomodation derivative contracts	$ 3	$ 3
Notional amount of the risk participation agreements	$ 3,677	2,838
Interest Rate Contract | Credit Risk
Derivative
Credit Risk Derivatives Average Remaining Life	3 years 7 months 6 days
Fair value of risk participation agreements	$ 8	5
Interest Rate Contract | Cash Flow Hedging
Derivative
Maximum length of time hedged in cash flow hedge	6 years 1 month
Deferred gains or losses, net of tax, on cash flow hedges were recorded in accumulated other comprehensive income	$ 16	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	7
Total collateral
Derivative
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	367	409
Derivative Liability, Fair Value, Amount Offset Against Collateral	401	$ 365
Amount of variation margin payment applied to derivative asset contracts	99
Amount of variation margin payment applied to derivative liability contracts	$ 130