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Reinsurance Ceded - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Effects Of Reinsurance [Line Items]        
Ceded loss and loss adjustment expenses incurred $ 562.7 $ 407.7 $ 628.2  
AIHL Re Limited Liability Company reinsurance of CapSpecialty Incorporated | Alleghany Corporation | Financial Guarantee        
Effects Of Reinsurance [Line Items]        
Guarantee amount related to reinsurance agreement 50.0      
AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company | Alleghany Corporation | Financial Guarantee        
Effects Of Reinsurance [Line Items]        
Guarantee amount related to reinsurance agreement       $ 150.0
CapSpecialty Incorporated | AIHL Re Limited Liability Company reinsurance of CapSpecialty Incorporated        
Effects Of Reinsurance [Line Items]        
Reinsurance coverage before co-participation $ 50.0      
Reinsurance Segment        
Effects Of Reinsurance [Line Items]        
Retrocession protection term 1 year      
Term of catastrophe bonds 4 years