Investments in Partially Owned Entities (Management, Development, Leasing and Other Agreements) (Details) - Alexanders Inc	12 Months Ended
	Dec. 31, 2019 USD ($) $ / ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Managment Agreement [Abstract]
Management fee income	$ 2,800,000
Development fee percent	6.00%
Leasing Agreement [Abstract]
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%
Rego Park 2 Property
Managment Agreement [Abstract]
Property management fee agreement percentage of income	2.00%
Lexington Avenue 731
Managment Agreement [Abstract]
Property management fee agreement price per square foot | $ / ft²	0.50
Common Area 731 Lexington Avenue
Managment Agreement [Abstract]
Management fee income	$ 324,000
Property management fee agreement escalation percentage of income	3.00%
Building Maintenance Service
Other Agreements [Abstract]
Fee income recognized by equity method investees for building maintenance services	$ 3,613,000	$ 2,705,000	$ 2,678,000