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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Concentration Risk [Line Items] |  |  |  |
Reinsurance recoverable for paid and unpaid claims and claims adjustments |  | $ 7,064 | $ 6,564 | $ 5,881
Shareholders' equity | Reinsurer concentration risk | Largest balance due from any one carrier |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage |  | 7.20% | 9.00% |
AM Best "A-" Or Better Rating | Reinsurance recoverable | Reinsurer concentration risk |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage |  | 66.80% | 68.80% |
Rating Below A- | Reinsurance recoverable | Reinsurer concentration risk | All Other Carriers |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage |  | 0.10% | 0.10% |
No AM Best rating | Reinsurance recoverable | Reinsurer concentration risk | Reinsurance trusts or letters of credit |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage |  | 95.00% | 95.00% |
No AM Best rating | Reinsurance recoverable | Reinsurer concentration risk | All Other Carriers |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage | [1] | 33.10% | 31.10% |
[1] | At December 31, 2023 and 2022 period, over 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively

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