Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 67,925us-gaap_ProfitLoss	$ 56,915us-gaap_ProfitLoss	$ 674,993us-gaap_ProfitLoss
Other comprehensive income:
Change in unrealized net gain on available-for-sale securities	13,124	316	1,206
Change in value of interest rate cap	(189)
Comprehensive income	80,860	57,231	676,199
Less comprehensive income attributable to the noncontrolling interest	0	0	(606)
Comprehensive income attributable to Alexander's	$ 80,860us-gaap_ComprehensiveIncomeNetOfTax	$ 57,231us-gaap_ComprehensiveIncomeNetOfTax	$ 675,593us-gaap_ComprehensiveIncomeNetOfTax

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income (loss) and other comprehensive income (loss), attributable to noncontrolling interests. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities, attributable to parent entity. Excludes amounts related to other than temporary impairment (OTTI) loss.

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