Document and Entity Information	9 Months Ended
	Sep. 30, 2011
Document and Entity Information [Abstract]
Entity Registrant Name	ALEXANDERS INC
Entity Central Index Key	0000003499
Document Type	10-Q
Document Period End Date	Sep. 30, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding	5,105,936
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes