Summary of Significant Accounting Policies (Details 1) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 200,876								$ 200,876
Reconciliation of Net Income to Estimated Taxable Income [Abstract]
Net income attributable to Alexander's	$ 23,572	$ 18,172	$ 17,341	$ 17,822	$ 18,161	$ 17,692	$ 16,828	$ 15,244	76,907	$ 67,925	$ 56,915
Straight-line rent adjustments									(1,418)	(2,538)	(3,707)
Depreciation and amortization timing differences									2,477	2,283	2,134
Reversal of income tax liability									0	(420)	(206)
Other									751	765	(2,186)
Estimated taxable income									$ 78,717	$ 68,015	$ 52,950
Ordinary Income [Member]
Tax Treatment Of Dividend [Line Items]
Payments Of Dividends Net Percent									97.30%
Long Term Capital Gain [Member]
Tax Treatment Of Dividend [Line Items]
Payments Of Dividends Net Percent									2.70%