Investments - Securities Accounted for As Available-for-Sale Included in Investments (Parenthetical) (Detail) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2021 |
Jun. 30, 2020 |
Jun. 30, 2021 |
Jun. 30, 2020 |
|
| Fair Value Disclosures [Abstract] | ||||
| Gross unrealized gain included in accumulated other comprehensive gain/(loss) | $ 273 | $ 631 | ||
| Gross unrealized loss included in accumulated other comprehensive gain/(loss) | 387 | |||
| Available-for-sale securities sold | $ 3,220 | $ 2,040 | 13,420 | 3,490 |
| Realized gains on sales of available-for-sale securities | $ 3 | $ 54 | $ 133 | $ 319 |