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Quarterly Operating Results (Unaudited) - Catastrophe losses (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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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
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |
Pretax catastrophe losses | $ (38) | $ (57) | $ (26) | $ (9) | $ (15) | $ (22) | $ (12) | $ (12) | $ (130) | $ (61)

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