Derivatives (Derivative Financial Instruments - Additional Information) (Detail) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ 306	$ 409
Valuation adjustments related to the credit risk associated with counterparties of customer accomodation derivative contracts	3	3
Derivative Liability, Fair Value, Amount Offset Against Collateral	490	365
Notional amount of the risk participation agreements	3,931	2,838
Amount of variation margin payment applied to derivative asset contracts	91
Amount of variation margin payment applied to derivative liability contracts	$ 143
Interest Rate Contract | Credit Risk
Derivative
Credit Risk Derivatives Average Remaining Life	3 years 8 months 12 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	5 years 10 months
Deferred gains or losses, net of tax, on cash flow hedges were recorded in accumulated other comprehensive income	$ 33	$ 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	$ 20