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Concentration of Credit Risk - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Reinsurance [Line Items] |  |  |
Unpaid losses | $ 3,195.2 | $ 2,319.8 | $ 2,077.6
Reinsurance Recoverables, Uncollateralized | 2,010.0 | 1,817.5 |
Underwriting premiums (net of allowance for expected credit losses of $34.0 — $Nil) | 1,279.8 | 1,318.4 |
Due for settlement | 11.4 |  |
Premium Receivable, Allowance for Credit Loss | $ 34.0 | 23.0 |
Allowable holdings of a single issue or issuer, percentage | 2.00% |  |
Percentage of written premium major broker accounted minimum | 10.00% |  |
Reinsurance Recoverable, Allowance for Credit Loss | $ 3.8 | $ 3.7 |
AM Best, A plus Rating | Standard & Poor's, A plus Rating | Everest Re |  |  |
Reinsurance [Line Items] |  |  |
Concentration risk percentage | 13.40% | 11.90% |
AM Best, A plus Rating | Standard & Poor's, AA- Rating | Munich Re |  |  |
Reinsurance [Line Items] |  |  |
Concentration risk percentage | 11.00% | 15.10% |
AM Best, A Rating | Standard & Poor's, A plus Rating | Lloyds |  |  |
Reinsurance [Line Items] |  |  |
Concentration risk percentage | 9.20% | 8.90% |

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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13531-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 + Details
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