Investments - Securities Accounted for As Available-for-Sale Included in Investments (Parenthetical) (Detail) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|---|
Sep. 30, 2021 |
Sep. 30, 2020 |
Sep. 30, 2021 |
Sep. 30, 2020 |
Dec. 31, 2020 |
|
| Fair Value Disclosures [Abstract] | |||||
| Gross unrealized gain included in accumulated other comprehensive gain/(loss) | $ 166 | $ 631 | |||
| Gross unrealized loss included in accumulated other comprehensive gain/(loss) | 432 | ||||
| Available-for-sale securities sold | $ 11,470 | $ 0 | 24,890 | $ 3,490 | |
| Realized gains on sales of available-for-sale securities | $ 54 | $ 187 | $ 319 | ||