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Reinsurance - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Ceded Credit Risk [Line Items]      
Reinsurance recoverables, allowance for doubtful accounts $ 4.7 $ 3.8 $ 4.1
Amount of collateral under reinsurance agreement 1,299.3 1,085.5  
Reinsurance recoverables 3,029.1 2,966.4  
Paid loss recoverables in insurance recoverables 190.6 494.6  
Losses and loss adjustment expenses 1,166.9 1,314.6 1,208.8
Net of amounts ceded to reinsurers 958.8 829.6 $ 941.3
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments | Reinsurer Concentration Risk      
Ceded Credit Risk [Line Items]      
Reinsurance recoverables 1,244.3 247.8  
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments | Ceded Credit Risk, Unsecured | Reinsurer Concentration Risk      
Ceded Credit Risk [Line Items]      
Reinsurance recoverables $ 564.0 $ 0.0