Regulation and Capital Adequacy - Risk-based Capital (Parenthetical) (Detail) - USD ($) $ in Millions |
9 Months Ended | 12 Months Ended |
|---|---|---|
Sep. 30, 2021 |
Dec. 31, 2020 |
|
| Regulation And Capital Adequacy [Abstract] | ||
| CECL adoption, percentage of increase in regulatory capital to increase in allowance for credit loss | 25.00% | 25.00% |
| Deferred tax liabilities associated with goodwill | $ 674 | $ 680 |
| Deferred tax liabilities associated with identifiable intangible assets | $ 19 | $ 29 |
| Subordinated debt maturity period | 5 years | 5 years |