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Concentration of Credit Risk - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Reinsurance [Line Items] |  |  |
Unpaid losses | $ 2,077.6 | $ 1,515.2 | $ 560.7
Reinsurance Recoverables, Uncollateralized | 1,497.8 | 1,001.9 |
Underwriting premiums | 1,459.3 | 1,496.5 |
Due for settlement | 12.4 |  |
Premiums Receivable, Allowance for Doubtful Accounts | $ 16.2 | $ 5.2 |
Allowable holdings of a single issue or issuer, percentage | 2.00% |  |
Percentage of written premium major broker accounted minimum | 10.00% |  |
AM Best, A plus Rating | Standard & Poor's, A plus Rating | Everest Re |  |  |
Reinsurance [Line Items] |  |  |
Concentration Risk, Percentage |  | 7.70% |
AM Best, A plus Rating | Standard & Poor's, AA- Rating | Munich Re |  |  |
Reinsurance [Line Items] |  |  |
Concentration Risk, Percentage | 15.70% | 17.00% |
AM Best, A plus Rating | Standard & Poor's, AA- Rating | Everest Re |  |  |
Reinsurance [Line Items] |  |  |
Concentration Risk, Percentage | 10.20% |  |
AM Best, A Rating | Standard & Poor's, A plus Rating | Lloyds |  |  |
Reinsurance [Line Items] |  |  |
Concentration Risk, Percentage | 10.20% | 13.80% |

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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=109250915&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=109250915&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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