Related Party Transactions (Details) (USD $)	12 Months Ended							0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Alexanders Inc [Member]	Dec. 31, 2012 Interstate Properties [Member]	Dec. 31, 2011 Interstate Properties [Member]	Dec. 31, 2010 Interstate Properties [Member]	Mar. 08, 2012 Board Of Directors Chairman [Member]
Related Party Transaction [Line Items]
Proceeds from the repayment of loan to officer	$ 13,123,000	$ 13,123,000	$ 0					$ 13,122,500
Loan to officer	0	13,123,000	0
Percentage of Company Owned by Related Party					6.50%
Percentage of Affiliated Entity Owned By Company				32.40%
Percentage Of Affiliated Entity Owned By Related Parties				26.30%
Fee And Other Income From Related Parties					$ 794,000	$ 787,000	$ 815,000
Related Party Transaction Annual Fee Percentage					4.00%
Related Party Transaction Description Of Transaction					We manage and lease the real estate assets of Interstate pursuant to a management agreement for which we receive an annual fee equal to 4% of annual base rent and percentage rent. The management agreement has a term of one year and is automatically renewable unless terminated by either of the parties on 60 days’ notice at the end of the term. We believe, based upon comparable fees charged by other real estate companies, that the management agreement terms are fair to us.