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Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Jan. 01, 2018
Sep. 12, 2017
Aug. 01, 2017
Apr. 28, 2017
Dec. 22, 2016
Apr. 15, 2016
Significant Accounting Policies [Line Items]              
Par value of common stock outstanding $ 1.00            
Retained Earnings              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements $ (12,899)            
Accumulated Other Comprehensive Income              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements [1] 12,899            
Unrealized Appreciation of Investments              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements [1] 12,900            
Accounting Standards Update 2017-08 | Retained Earnings              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements (13,000)            
Accounting Standards Update 2017-08 | Accumulated Other Comprehensive Income              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements $ 13,000            
Minimum              
Significant Accounting Policies [Line Items]              
Useful life of property and equipment 3 years            
Maximum              
Significant Accounting Policies [Line Items]              
Useful life of property and equipment 20 years            
Subsequent Event | Accounting Standards Update 2016-01 | Retained Earnings              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements   $ 734,000          
Subsequent Event | Accounting Standards Update 2016-01 | Unrealized Appreciation of Investments              
Significant Accounting Policies [Line Items]              
Cumulative effective of adoption of new accounting pronouncements   $ (734,000)          
Wilbert Funeral Services, Inc              
Significant Accounting Policies [Line Items]              
Equity interest percentage 45.00%            
Cost of equity investment       $ 72,300      
ORX              
Significant Accounting Policies [Line Items]              
Disposal date Dec. 23, 2016            
Equity interest percentage           15.00%  
Pacific Comp              
Significant Accounting Policies [Line Items]              
Cash consideration from sale     $ 158,000        
Disposal date Dec. 31, 2017            
Assets disposed of $ 442,000            
Liabilities disposed of 316,000            
Gain on sale of business, after tax 16,000            
Alleghany Corporation | Financial Guarantee | AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company              
Significant Accounting Policies [Line Items]              
Guaranteed obligation, aggregate limit $ 150,000            
Kentucky Trailer              
Significant Accounting Policies [Line Items]              
Ownership of interest held by noncontrolling partners 21.00%            
Integrated Project Services LLC              
Significant Accounting Policies [Line Items]              
Ownership of interest held by noncontrolling partners 16.00%            
Bourn & Koch, Inc.              
Significant Accounting Policies [Line Items]              
Ownership of interest held by noncontrolling partners 11.00%            
Jazwares, LLC              
Significant Accounting Policies [Line Items]              
Ownership of interest held by noncontrolling partners 20.00%            
WWSC Holdings, LLC              
Significant Accounting Policies [Line Items]              
Ownership of interest held by noncontrolling partners 20.00%            
Aihl Re Limited Liability Company | AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company              
Significant Accounting Policies [Line Items]              
Aggregate limit $ 150,000            
Final commutation and settlement date Dec. 31, 2024            
Jazwares, LLC              
Significant Accounting Policies [Line Items]              
Ownership interest acquired             50.00%
Ownership Percentage by Parent             80.00%
WWSC Holdings, LLC              
Significant Accounting Policies [Line Items]              
Ownership interest acquired         80.00%    
Business acquisition date Apr. 28, 2017            
[1] See Note 1(r) for additional information regarding Alleghany's adoption of new investment accounting guidance in 2017.