| Document and Entity Information | Sep. 14, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 14, 2020 |
| Entity Registrant Name | ALEXANDERS INC |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-06064 |
| Entity Tax Identification Number | 51-0100517 |
| Entity Address, Address Line One | 210 Route 4 East |
| Entity Address, City or Town | Paramus, |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 07652 |
| City Area Code | 201 |
| Local Phone Number | 587-8541 |
| Title of 12(b) Security | Common Stock, $1 par value per share |
| Trading Symbol | ALX |
| Security Exchange Name | NYSE |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000003499 |
| Amendment Flag | false |
