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Basis of Preparation and Significant Accounting Policies - Additional Information (Details)
$ in Millions
 12 Months Ended
Jan. 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
period
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Jan. 01, 2019
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      
Right-of-use operating lease assets   $ 93.5 $ 0.0   $ 88.1
Operating lease liabilities   $ 113.2     $ 95.5
Operating lease, weighted average discount rate (percent)   5.00%      
Operating lease charge   $ 18.0 0.0 $ 0.0  
Amortization of right-of-use operating lease assets   13.4 $ 0.0 0.0  
Interest on operating lease liability   $ 4.6      
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%      
Restatement Adjustment | Retained earnings          
Property, Plant and Equipment [Line Items]          
Retained earnings restatement [1]       $ 15.6  
Subsequent Event | Accounting Standards Update 2016-13 [Member]          
Property, Plant and Equipment [Line Items]          
Available-for-sale investment portfolio $ 0.5        
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease) (3.7)        
Subsequent Event | Accounting Standards Update 2016-13 [Member] | Retained earnings          
Property, Plant and Equipment [Line Items]          
Cumulative effect of new accounting principle in period of adoption $ 4.2        
[1] Reinsurance premiums payables and retained earnings have been restated by $15.6 million from January 1, 2017 to account for additional ceded premiums on excess of loss ceded reinsurance contracts for periods December 31, 2016 and prior.