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Reinsurance Reinsurance (Written Consolidated Property Casualty Insurance Premiums On Assumed and Ceded Business) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance |  |  |
Direct premiums earned | $ 7,801 | $ 7,056 | $ 6,366
Assumed premiums earned | 569 | 561 | 443
Ceded premiums earned | (412) | (392) | (331)
Incurred loss and loss expenses | 5,274 | 5,019 | 3,909
Consolidated Property And Casualty Insurance Entity |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance |  |  |
Direct premiums written | 7,784 | 7,002 | 6,229
Assumed premiums written | 597 | 619 | 515
Ceded premiums written | (335) | (314) | (265)
Net written premiums | 8,046 | 7,307 | 6,479
Direct premiums earned | 7,407 | 6,677 | 5,996
Assumed premiums earned | 569 | 561 | 443
Ceded premiums earned | (331) | (314) | (255)
Earned premiums | 7,645 | 6,924 | 6,184
Direct incurred loss and loss expenses | 4,843 | 4,495 | 3,352
Assumed incurred loss and loss expenses | 280 | 396 | 366
Ceded incurred loss and loss expenses | (165) | (175) | (122)
Incurred loss and loss expenses | $ 4,958 | $ 4,716 | $ 3,596

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