Investments in Partially Owned Entities (Alexander's Inc.) (Details) (Alexanders Inc [Member], USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Alexanders Inc [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Ownership common shares, investee (in shares)	1,654,068
Equity method ownership percentage	32.40%
Closing share price (in dollars per share)	$ 456.58
Equity Method Investment Market Value	$ 755,214,000
Excess of investee's fair value over carrying amount	623,071,000
Excess of investee's carrying amount over equity in net assets	41,048,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets	1,433,000,000		1,423,000,000
Liabilities	1,084,000,000		1,075,000,000
Equity	349,000,000		348,000,000
Total revenue	52,000,000	49,000,000
Net income attributable to investee	$ 18,000,000	$ 15,000,000