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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% |  |

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