Cover - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Jan. 31, 2023	Jun. 30, 2022
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2022
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
Entity Shell Company	false
Entity Public Float			$ 472,173
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 18, 2023.
Entity Central Index Key	0000003499
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Amendment Flag	false