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Investments in Partially Owned Entities (Alexander's Inc.) (Details)
1 Months Ended
Jun. 02, 2017
USD ($)
Extensions
May 31, 2017
USD ($)
Jun. 30, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
Equity Method Investments And Income From Equity Method Investments        
Notes And Loans Payable     $ 9,502,694,000 $ 9,278,263,000
Alexanders Inc        
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     $ 421.46  
Equity Method Investment Market Value     $ 697,124,000  
Excess of investee's fair value over carrying amount     570,494,000  
Excess of investee's carrying amount over equity in net assets     $ 39,468,000  
Alexanders Inc | Lexington Avenue 731 | Office        
Equity Method Investments And Income From Equity Method Investments        
Notes And Loans Payable $ 500,000,000 $ 300,000,000    
Debt Instrument, Basis Spread on Variable Rate 0.90% 0.95%    
Interest Rate, Effective     2.06%  
Description of variable rate basis LIBOR LIBOR    
Length Of Extension Available 1 year      
Number of loan extensions available | Extensions 4      
Debt Instrument Maturity 2020-06 2021-03    
Alexanders Inc | Lexington Avenue 731 | Office | Interest rate cap        
Equity Method Investments And Income From Equity Method Investments        
Notional amount $ 500,000,000      
Cap interest rate 6.00%