Derivatives and Hedging Activities - Carrying Amount of Hedged Items Currently Designated in a Hedging Relationship and Related Cumulative Hedging Adjustment (Parenthetical) (Detail) - Unsecured long-term borrowings [Member] - Derivative Contracts Accounted for as Hedges [Member] - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cumulative hedging adjustment	$ 5,250	$ 3,480
Addition Cumulative Hedging adjustments	$ 792	$ 425