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Reinsurance - Additional Information (Details) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 02, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year |  |  | $ 3,195.2 | $ 2,319.8 | $ 2,077.6
Plus component of annual investment management fee |  |  | 0.25 |  |
Reinsurance Recoverable, Allowance for Credit Loss |  |  | $ 3.8 | $ 3.7 |
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) |  |  | $ 0.1 |  |
Enstar |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year | [1] | $ 770.0 |  |  |
Second coverage |  | 250.0 |  |  |
Maximum | Enstar |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |
Reinsurance retention policy, amount retained |  | 4,575.0 |  |  |
Reinsurance retention policy, second amount retained |  | 5,065.0 |  |  |
Minimum | Enstar |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |
Reinsurance retention policy, amount retained |  | 3,805.0 |  |  |
Reinsurance retention policy, second amount retained |  | $ 4,815.0 |  |  |
[1] | Included within reinsurance recoverable for unpaid losses are $770.0 million of recoveries associated with the purchase of an adverse development cover.

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- Definition Reinsurance Retention Policy, Amount Retained, Second Tier + References No definition available. + Details
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- Definition Amount of increase (decrease) in allowance for credit loss on reinsurance recoverable. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447 + Details
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