Investments - Gross Realized Gains and Gross Realized Losses from Sales or Maturities of Available for Sale Securities (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Investments, Debt and Equity Securities [Abstract] | ||||
| Aggregate fair value of available for sale securities sold | $ 3,300 | $ 2,500 | $ 9,800 | $ 9,900 |
| Gross realized gains | 18 | 15 | 56 | 45 |
| Gross realized losses | $ 249 | $ 117 | $ 480 | $ 560 |