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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Percentage of comparable consolidated amounts 95.00% 95.00% 95.00%
Original maturity of cash equivalent 90 days    
Minimum period of underwriting workers? compensation insurance from which historical loss data utilize 39 years    
Experience rated commission and other operating costs $ 1.0 $ 1.7 $ 0.1
Advertising expense $ 0.3 $ 0.2 $ 0.2
Assessment Related premium Payment period 1 year    
Assessment Related loss Payment period 1 year    
Hannover Re and Tokio Millenium Re [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Reinsurance commutation recepit adjustment $ 6.3    
Pre-tax loss related to reinsurance commutation 1.5    
Hannover Re and Allianz Risk Transfer [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Reinsurance commutation recepit adjustment $ 9.8    
Buildings [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Estimated useful life of assets 39 years    
Maximum [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Short-term investments maturity period 1 year    
Maximum [Member] | Other Fixed Assets [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Estimated useful life of assets 7 years    
Minimum [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Short-term investments maturity period 90 days    
Minimum [Member] | Other Fixed Assets [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Estimated useful life of assets 3 years