Investments in Partially Owned Entities (Alexander's Inc.) (Details) - Alexanders Inc $ / shares in Units, $ in Thousands	Mar. 31, 2017 USD ($) $ / shares shares
Equity Method Investments And Income From Equity Method Investments
Ownership common shares, investee (in shares) | shares	1,654,068
Equity method ownership percentage	32.40%
Closing share price (in dollars per share) | $ / shares	$ 431.86
Equity Method Investment Market Value	$ 714,326
Excess of investee's fair value over carrying amount	586,418
Excess of investee's carrying amount over equity in net assets	$ 39,651