Derivatives and Hedging Activities - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) derivative rating	Dec. 31, 2017 USD ($)
Credit Derivatives [Line Items]
Derivative assets after effects of bilateral collateral and master netting agreements	$ 48
Derivative liabilities after effects of bilateral collateral and master netting agreements	(3)
Derivative assets not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	700
Derivative liabilities not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	446
Reclassify of after-tax net losses on derivative instruments from AOCI	$ 45
Length of time hedge in cash flow hedge	12 months
Reclassification of net losses related to terminated cash flow hedges from AOCI to income	$ 45
Maximum length of time over which forecasted transactions are hedged, years	10 years
Other Comprehensive income unrealized gain on derivatives arising during period net of tax	$ 43
Cash collateral netted against derivative assets	(116)	$ (23)
Collateral netted against derivative liabilities	304	150
Gross exposure on derivatives, after taking into account the effects of bilateral collateral and master netting agreements	148
Net exposure on derivatives, after taking into account, the effects of bilateral collateral and master netting agreements	215
Over-collateralization on derivatives to broker-dealers and banks, after the application of master netting agreements and collateral	205
Additional collateral held in the form of securities	10
Default reserve associated with uncollateralized contracts	4
Gross exposure on derivatives after taking into account effects of master netting agreements	592
Net exposure on derivatives with clients after application of master netting agreements collateral and related reserve	534
Banking Subsidiary [Member]
Credit Derivatives [Line Items]
Net liability position totaled	335
Derivative assets included in net liability position	144
Derivative liabilities included in net liability position	479
Cash and securities collateral posted	$ 317
Number of credit risk derivatives held | derivative	0
Additional collateral amount	$ 1	0
Banking Subsidiary [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Number of ratings above noninvestment | rating	4
Banking Subsidiary [Member] | Maximum [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts (less than for the $1 million)	$ 2	$ 12