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Quarterly Operating Results (Unaudited) - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 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 | Jun. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities |  |  |  |  |  |  |  |  |  | $ 289 | $ 287 | $ (266)
Pretax catastrophe losses | $ 38 | $ 57 | $ 26 | $ 9 | $ 15 | $ 22 | $ 12 | $ 12 |  | 130 | 61 |
Pretax COVID-19 related losses |  |  | 105 | 10 |  |  |  |  |  | 115 |  |
Other Security Investments |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities | 384 | 15 | 2 | (16) | (2) | (3) | 12 | 2 |  | 385 | 9 |
Property and casualty insurance | Other lines |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in asbestos and environmental reserves |  | 47 |  |  |  | 18 |  |  |  | 47 | 18 | 18
Property and casualty insurance | Other lines | Neon Capital Limited |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Pretax catastrophe losses |  |  |  |  |  |  |  |  |  | 37 | 13 |
Pretax COVID-19 related losses |  |  |  |  |  |  |  |  | $ 20 | 20 |  |
Other |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Write off of deferred debt issuance cost due to the retirement of debt | 5 |  |  |  | 5 |  |  |  |  |  |  |
Other | Former railroad and manufacturing operations |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in asbestos and environmental reserves |  | 21 |  |  |  | 11 |  |  |  |  |  |
Annuity | Other Security Investments |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities | 369 |  |  |  |  |  |  |  |  |  |  |
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 |
Total fixed maturities |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) on securities |  |  |  |  |  |  |  |  |  | $ 422 | $ (3) | $ (20)

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