XML 16 R1.htm IDEA: XBRL DOCUMENT v3.24.0.1
Cover - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Jan. 31, 2024
Jun. 30, 2023
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2023    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-06064    
Entity Registrant Name ALEXANDERS INC    
Entity Incorporation, State or Country Code DE    
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    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 390,771
Entity Common Stock, Shares Outstanding   5,107,290  
Documents Incorporated by Reference
Part III: Portions of the Proxy Statement for the Annual Meeting of Stockholders to be held on May 23, 2024.
    
Entity Central Index Key 0000003499    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Amendment Flag false