Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Jul. 31, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		5,107,290
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Entity Central Index Key	0000003499
Current Fiscal Year End Date	--12-31