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Quarterly Operating Results (Unaudited) - Realized gains (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities |  |  |  |  |  |  |  |  | $ 289 | $ 287 | $ (266)
Other Security Investments |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities | $ 384 | $ 15 | $ 2 | $ (16) | $ (2) | $ (3) | $ 12 | $ 2 | 385 | 9 |
Equity securities |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities |  |  |  |  |  |  |  |  | (96) | 277 | $ (265)
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities | $ 207 | $ 30 | $ 202 | $ (535) | $ 67 | $ (15) | $ 44 | $ 182 | $ (96) | $ 278 |

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