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 |
Dec. 31, 2020 |
Dec. 31, 2019 |
|---|---|---|
| Derivative Instruments and Hedging Activities Disclosures [Line Items] | ||
| Cumulative hedging adjustment | $ 6,340 | $ 3,480 |
| Addition Cumulative Hedging adjustments | $ 489 | $ 425 |