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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Ceded Credit Risk [Line Items] |  |  |
Reinsurance recoverable on unpaid and paid losses and LAE |  | $ 5,358,852 | $ 4,500,802
Reinsurance recoverable | Reinsurer concentration risk | AM Best A minus Or Better Rating |  |  |
Ceded Credit Risk [Line Items] |  |  |
Concentration risk percentage |  | 69.10% | 63.90%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated |  |  |
Ceded Credit Risk [Line Items] |  |  |
Concentration risk percentage | [1] | 30.90% | 36.10%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | Reinsurance trusts or letters of credit |  |  |
Ceded Credit Risk [Line Items] |  |  |
Concentration risk percentage |  | 93.00% | 94.00%
Stockholders' equity | Reinsurer concentration risk | Single Reinsurer |  |  |
Ceded Credit Risk [Line Items] |  |  |
Concentration risk percentage |  | 6.50% | 1.80%
[1] | At September 30, 2021 and December 31, 2020 over 93% and 94% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively

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- Definition Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919253-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953401-111524 + Details
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