Significant Accounting Policies (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Real Estate [Abstract]
Interest Paid, Capitalized											$ 1,269,000
Cash and Cash Equivalents [Abstract]
Cash And Cash Equivalent Maturity Maximum									3 months
Short-term Investments [Abstract]
Available-for-sale Securities, Debt Maturity, Date Range, Low									3 months
Available-for-sale Securities, Debt Maturity, Date Range, High									6 months
Allowance For Doubtful Accounts [Abstract]
Allowance for Doubtful Accounts Receivable	2,219,000				1,039,000				2,219,000	1,039,000
Income Taxes [Abstract]
Real Estate Investment Trust Distributable Income Policy									90.00%
Internal Taxable Income Distribution Policy									100.00%
Differences Between Book and Tax Basis	184,806,000								184,806,000
Deferred Tax Assets (Liabilities), Net	0				0				0	0
Reconciliation of Net Income to Estimated Taxable Income [Abstract]
Net income attributable to Alexander's	617,157,000	18,856,000	18,892,000	19,482,000	20,634,000	20,425,000	20,157,000	18,207,000	674,387,000	79,423,000	66,429,000
Additional tax gain on the sale of real estate									23,928,000	0	0
Straight-lining of rental income									(4,475,000)	(12,609,000)	(15,182,000)
Depreciation and amortization timing differences									910,000	1,263,000	602,000
Tax Adjustments, Settlements, and Unusual Provisions									0	0	(3,162,000)
Interest expense									29,000	(2,425,000)	0
Other									4,396,000	(3,429,000)	6,245,000
Taxable income before NOL									699,175,000	62,223,000	54,932,000
NOL carried forward	0				0				0	0	(16,939,000)
Taxable income/(NOL)	$ 699,175,000				$ 62,223,000				$ 699,175,000	$ 62,223,000	$ 37,993,000
Maximum [Member]
Real Estate Properties [Line Items]
Property plant and equipment estimated useful lives									40
Minimum [Member]
Real Estate Properties [Line Items]
Property plant and equipment estimated useful lives									5
Rego Park 1 Shopping [Member]
Real Estate Properties [Line Items]
Percentage of cash mortgage collateralized	100.00%								100.00%