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Segments of Business - Additional Information (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2020
USD ($)
Segment
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Segment Reporting Information [Line Items]      
Number of reportable segments | Segment 3    
Gross premiums written $ 7,329,400 $ 6,656,400 $ 5,895,600
Net premiums earned $ 6,000,161 $ 5,478,143 $ 4,976,190
Reinsurance Segment | Largest Broker Concentration Risk | Gross Premiums Written      
Segment Reporting Information [Line Items]      
Percentage of gross premiums written 27.00% 26.00% 26.00%
Reinsurance Segment | Second Largest Broker Concentration Risk | Gross Premiums Written      
Segment Reporting Information [Line Items]      
Percentage of gross premiums written 19.00% 17.00% 18.00%
Reinsurance Segment | Third Largest Broker Concentration Risk | Gross Premiums Written      
Segment Reporting Information [Line Items]      
Percentage of gross premiums written 14.00% 16.00% 15.00%
Reinsurance Segment | Large Whole Account Quota Share Treaty | Gross Premiums Written      
Segment Reporting Information [Line Items]      
Percentage of gross premiums written 13.00% 14.00% 17.00%
Operating Segments      
Segment Reporting Information [Line Items]      
Gross premiums written $ 7,363,000 $ 6,684,100 $ 5,921,800
Net premiums earned 6,000,200 5,478,100 4,976,200
Operating Segments | Alleghany Capital Corporation Segment      
Segment Reporting Information [Line Items]      
Gross premiums written [1] 0 0 0
Net premiums earned [1] 0 0 0
Operating Segments | Reinsurance Segment      
Segment Reporting Information [Line Items]      
Gross premiums written 5,237,300 4,945,700 4,451,000
Net premiums earned 4,644,700 4,327,000 3,939,000
Operating Segments | Reinsurance Segment | Non-US      
Segment Reporting Information [Line Items]      
Gross premiums written 1,700,000 1,700,000 1,600,000
Net premiums earned 1,600,000 1,500,000 1,400,000
Operating Segments | Reinsurance Segment | U.K.      
Segment Reporting Information [Line Items]      
Net premiums earned $ 699,600 $ 615,600 $ 583,800
[1] Excludes certain minor, legacy investments that were previously reflected in Alleghany Capital in 2018 and prior periods, to align with management’s view of reportable segments.