000001 - Document and Entity Information (USD $) In Thousands, except Share data	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Document and Entity Information [Abstract]
Entity Registrant Name	ALEXANDERS INC
Entity Central Index Key	0000003499
Document Type	10-Q
Document Period End Date	Jun. 30, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q2
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding	5,105,936
Entity Public Float		$ 622,802
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes