Derivatives and Hedging Activities - Carrying Amount of Hedged Items Currently Designated in a Hedging Relationship and Related Cumulative Hedging Adjustment (Detail) - Derivative Contracts Accounted for as Hedges [Member] - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Deposits at Fair Value [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount	$ 17,963	$ 19,634
Cumulative Hedging Adjustment	806	200
Unsecured short-term borrowings [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount	6,685	6,008
Cumulative Hedging Adjustment	45	28
Unsecured long-term borrowings [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Carrying Amount	116,349	87,874
Cumulative Hedging Adjustment	$ 12,799	$ 7,292