Investments - Securities Accounted for As Available-for-Sale Included in Investments (Parenthetical) (Detail) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
|
| Fair Value Disclosures [Abstract] | ||
| Gross unrealized gain included in accumulated other comprehensive gain/(loss) | $ 631 | $ 137 |
| Available-for-sale securities sold | 4,490 | 9,580 |
| Realized gains on sales of available-for-sale securities | $ 319 | $ 181 |