Significant Accounting Policies (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real Estate [Abstract]
Capitalized Interest									$ 0	$ 1,269,000	$ 3,452,000
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	1,039,000				1,047,000				1,039,000	1,047,000
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	209,775,000								209,775,000
Deferred Tax Assets (Liabilities), Net	0				0				0	0
Reconciliation of Net Income to Estimated Taxable Income [Abstract]
Net income attributable to Alexander's	20,634,000	20,425,000	20,157,000	18,207,000	17,891,000	17,875,000	15,549,000	15,114,000	79,423,000	66,429,000	132,190,000
Straight-lining of rental income									(12,609,000)	(15,182,000)	(23,381,000)
Depreciation and amortization timing differences									1,263,000	602,000	1,385,000
Reversal of liablity for income taxes									0	(3,162,000)	(37,307,000)
Interest expense									(2,425,000)	0	(107,000)
Stock appreciation rights compensation expense									0	0	(57,113,000)
Other									(3,429,000)	6,245,000	(3,395,000)
Taxable income before NOL									62,223,000	54,932,000	12,272,000
NOL carried forward	0				(16,939,000)				0	(16,939,000)	(29,211,000)
Taxable income/(NOL)									$ 62,223,000	$ 37,993,000	$ (16,939,000)
Rego Park I Shopping Center
Restricted Cash and Cash Equivalents Items [Line Items]
Percentage of cash mortgage collateralized	100.00%								100.00%