| Document and Entity Information | Nov. 03, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | ALLEGHANY CORP /DE |
| Amendment Flag | false |
| Entity Central Index Key | 0000775368 |
| Document Type | 8-K |
| Document Period End Date | Nov. 3, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-9371 |
| Entity Tax Identification Number | 51-0283071 |
| Entity Address, Address Line One | 1411 Broadway |
| Entity Address, Address Line Two | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $1.00 par value |
| Trading Symbol | Y |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
