Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 25, 2021
Cover [Abstract]
Amendment Flag	false
Document Type	10-Q
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
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		13,927,263
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Trading Symbol	Y
Title of 12(b) Security	Common Stock
Security Exchange Name	NYSE
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	NY
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10018
City Area Code	212
Local Phone Number	752-1356
Document Quarterly Report	true
Document Transition Report	false