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Basis of Preparation and Significant Accounting Policies - Additional Information (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
period
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Property, Plant and Equipment [Line Items]      
Threshold Period of Value Decline in Equity Securities to be considered Other than Temporary Impairment 12 months    
Non-Payment Of Dividends, Number Of Periods | period 6    
Amortization of right-of-use operating lease assets $ 11.8 $ 13.4 $ 0.0
Available-for-sale investment, allowance for credit loss, increase (decrease) (0.4)    
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) 0.1    
Unpaid losses (1) (net of allowance for expected credit losses of $3.8 — $Nil) $ 3.8 3.7  
Computer Equipment | Minimum      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 3 years    
Computer Equipment | Maximum      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 5 years    
Furniture and Fixtures      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 4 years    
Leasehold Improvements      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 15 years    
Fixed maturities      
Property, Plant and Equipment [Line Items]      
Percentage of Value Decline in Securities to be considered Other than Temporary Impairment 20.00%    
Accounting Standards Update 2016-13      
Property, Plant and Equipment [Line Items]      
Available-for-sale investment, allowance for credit loss, increase (decrease)   0.6  
Accounting Standards Update 2016-13 | Retained earnings      
Property, Plant and Equipment [Line Items]      
Cumulative Effect on Retained Earnings, before Tax $ (4.3) [1] $ 0.0 $ 0.0
[1] The $4.3 million relates to the cumulative effect-adjustment to opening retained earnings as a result of the recognition of current expected credit losses (‘CECL’) in the Company’s available-for-sale investment portfolio and reinsurance recoverables following the adoption of ASU 2016-13.