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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
Ceded Credit Risk [Line Items] |  |  |  |  |
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses |  | $ 7,509 | $ 7,064 | $ 6,612 | $ 6,564
Reinsurance recoverable | Reinsurer concentration risk | AM Best A minus Or Better Rating |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 66.60% | 66.80% |  |
Reinsurance recoverable | Reinsurer concentration risk | AM Best Rating Below A Minus | All Other Carriers |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 0.00% | 0.10% |  |
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | All Other Carriers |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage | [1] | 33.40% | 33.10% |  |
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | Ceded credit risk, secured |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 94.00% | 95.00% |  |
Stockholders' equity | Reinsurer concentration risk | Largest Balance Due From Any One Carrier |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 7.10% | 7.20% |  |
[1] | At March 31, 2024 and December 31, 2023 over 94% and 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively

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