Derivatives and Hedge Accounting Activities (Schedule of Amounts Recorded on Balance Sheet Related to Cumulative Basis Adjustments for Fair Value Hedges) (Parenthetical) (Detail) - Designated as Hedging Instrument - Fair Value Hedging - Long-term Debt - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Discontinued hedging liability	$ (1,153)	$ (397)
Hedging adjustments on discontinued hedging relationships	$ (3)	$ 3