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Reinsurance - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Mar. 02, 2020
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]          
Plus reinsurance recoverable on unpaid losses at the end of the year   $ 3,298.1 $ 3,195.2 $ 2,319.8  
Deferred Gain on Reinsurance   58.3 0.0 0.0  
Amortization of deferred gains on reinsurance   9.8      
Reinsurance Recoverable, Allowance for Credit Loss   3.3 3.8 3.7  
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease)   (0.5) 0.1    
ADC          
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]          
Plus reinsurance recoverable on unpaid losses at the end of the year     2,518.3 2,425.2 $ 2,319.8
Reinsurance recoverables, premium paid and recognized   $ 68.1 $ 0.0 $ 0.0  
Enstar          
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]          
Plus reinsurance recoverable on unpaid losses at the end of the year $ 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