Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash and Cash Equivalents [Abstract]
Cash And Cash Equivalent Maturity Maximum									3 months
Allowance For Doubtful Accounts [Abstract]
Allowance for Doubtful Accounts Receivable	$ 1,993				$ 2,219				$ 1,993	$ 2,219
Tax Treatment Of Dividend [Line Items]
Real Estate Investment Trust Distributable Income Policy (in percentage)									90.00%
Internal Taxable Income Distribution Policy (in percentage)									100.00%
Differences Between Book and Tax Basis	189,757								189,757
Reconciliation of Net Income to Estimated Taxable Income [Abstract]
Net income attributable to Alexander's	15,790	13,824	13,139	14,162	617,157	18,856	18,892	19,482	56,915	674,387	79,423
Straight-line rent adjustments									(3,707)	(4,475)	(12,609)
Depreciation and amortization timing differences									2,134	910	1,263
Interest expense									27	29	(2,425)
Reversal of liablity for income taxes									(206)	0	0
Additional tax gain on sale of the Kings Plaza Regional Shopping Center									0	23,928	0
Other									(2,213)	4,396	(3,429)
Estimated taxable income									$ 52,950	$ 699,175	$ 62,223
Ordinary Income [Member]
Tax Treatment Of Dividend [Line Items]
Payments Of Dividends Net Percent										10.90%
Long Term Capital Gain [Member]
Tax Treatment Of Dividend [Line Items]
Payments Of Dividends Net Percent										89.10%
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									3
Rego Park I [Member]
Real Estate Properties [Line Items]
Percentage of cash mortgage collateralized	100.00%								100.00%