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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2022 | Dec. 31, 2021 | Mar. 31, 2021 | Dec. 31, 2020
Ceded Credit Risk [Line Items] |  |  |  |  |
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses |  | $ 5,941,000 | $ 5,880,735 | $ 4,041,076 | $ 4,500,802
Reinsurance recoverable | Reinsurer concentration risk | AM Best A minus Or Better Rating |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 69.50% | 69.70% |  |
Reinsurance recoverable | Reinsurer concentration risk | AM Best Rating Below A Minus | All Other Carriers |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 0.10% | 0.10% |  |
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | All Other Carriers |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage | [1] | 30.40% | 30.20% |  |
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | Reinsurance trusts or letters of credit |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 93.00% | 91.00% |  |
Stockholders' equity | Reinsurer concentration risk | Largest Balance Due From Any One Carrier |  |  |  |  |
Ceded Credit Risk [Line Items] |  |  |  |  |
Concentration risk percentage |  | 7.20% | 6.70% |  |
[1] | At March 31, 2022 and December 31, 2021 over 93% and 91% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively

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- Definition Amount, after 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, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 825 -Section 50 -Paragraph 1B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123600520&loc=SL75241803-196195 + Details
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