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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Concentration Risk [Line Items] |  |  |
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses (net of allowance for credit losses: $11,636 and $1,364) |  | $ 4,500,802 | $ 4,346,816
Concentration risk percentage |  | 10.00% |
Stockholders' equity | Reinsurer concentration risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk percentage |  | 1.80% | 1.70%
AM Best A minus Or Better Rating | Reinsurance recoverable | Credit concentration risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk percentage |  | 63.90% | 61.20%
No AM Best rating | Reinsurance recoverable | Credit concentration risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk percentage | [1] | 36.10% | 38.80%
No AM Best rating | Reinsurance recoverable | Credit concentration risk | Reinsurance trusts or letters of credit |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk percentage |  | 94.00% |
[1] | Over 94% of such amount is collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other

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