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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.