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Investments in Partially Owned Entities (Management, Development, Leasing and Other Agreements for Alexander's) (Details)
12 Months Ended
Dec. 31, 2021
USD ($)
$ / ft²
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
BMS cleaning fees      
Schedule of Equity Method Investments [Line Items]      
Fee income recognized by equity method investees for building maintenance services $ 4,234,000 $ 3,613,000 $ 3,613,000
Alexanders Inc      
Schedule of Equity Method Investments [Line Items]      
Management fee income $ 2,800,000    
Development fee percent 6.00%    
Lease fee percentage of rent one to ten years 3.00%    
Lease fee percentage of rent eleven to twenty years 2.00%    
Lease fee percentage of rent twenty first to thirty years 1.00%    
Percentage increase lease fee if broker used 1.00%    
Percentage commissions on sale of assets under fifty million 3.00%    
Asset sale commission threshold $ 50,000,000    
Percentage commissions on sale of assets over fifty million 1.00%    
Alexanders Inc | Rego Park 2 Property      
Schedule of Equity Method Investments [Line Items]      
Property management fee agreement percentage of income 2.00%    
Alexanders Inc | Lexington Avenue 731      
Schedule of Equity Method Investments [Line Items]      
Property management fee agreement price per square foot | $ / ft² 0.50    
Alexanders Inc | Common Area 731 Lexington Avenue      
Schedule of Equity Method Investments [Line Items]      
Management fee income $ 344,000    
Property management fee agreement escalation percentage of income 3.00%