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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 09, 2020
Jun. 30, 2019
Cover [Abstract]      
Amendment Flag false    
Document Type 10-K    
Document Period End Date Dec. 31, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Entity Registrant Name ALLEGHANY CORP /DE    
Entity Central Index Key 0000775368    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   14,350,883  
Entity Shell Company false    
Entity Small Business false    
Entity Emerging Growth Company false    
Title of 12(b) Security Common Stock, $1.00 par value    
Security Exchange Name NYSE    
Trading Symbol Y    
Entity File Number 1-9371    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 51-0283071    
Entity Address, Address Line One 1411 Broadway, 34th Floor    
Entity Address, City or Town New York    
Entity Address, State or Province NY    
Entity Address, Postal Zip Code 10018    
City Area Code 212    
Local Phone Number 752-1356    
Document Annual Report true    
Document Transition Report false    
Entity Public Float     $ 9,902,910,301
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer Yes    
Documents Incorporated by Reference

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive proxy statement relating to the Annual Meeting of Stockholders of Alleghany Corporation to be held on April 24, 2020 to be filed with the Securities and Exchange Commission pursuant to Regulation 14A are incorporated by reference into Part III of this Annual Report on Form 10-K.