Organization and Business - (Narrative) (Details) ft² in Thousands	12 Months Ended
	Dec. 31, 2018 ft² room property building unit
Real estate fund investments
Real Estate Properties
Equity method ownership percentage	25.00%
Alexanders Inc
Real Estate Properties
Equity method ownership percentage	32.40%
New York | Manhattan | Office
Real Estate Properties
Square footage of real estate property	19,900
Number of real estate properties | property	36
New York | Manhattan | Retail
Real Estate Properties
Square footage of real estate property	2,600
Number of real estate properties | property	71
New York | Manhattan | Residential Properties
Real Estate Properties
Number of real estate properties | property	11
Number of units in real estate property (in number of residential units) | unit	1,999
New York | Manhattan | Hotel Pennsylvania
Real Estate Properties
Number of units in real estate property (in number of residential units) | room	1,700
New York | Alexanders Inc | New York City Metropolitan Area
Real Estate Properties
Number of real estate properties | property	7
Equity method ownership percentage	32.40%
New York | Alexanders Inc | New York City Metropolitan Area | Lexington Avenue 731
Real Estate Properties
Square footage of real estate property	1,300
Other | Real estate fund investments
Real Estate Properties
Equity method ownership percentage	25.00%
Other | Chicago | the Mart, Chicago
Real Estate Properties
Square footage of real estate property	3,700
Other | San Francisco | 555 California Street | Office
Real Estate Properties
Square footage of real estate property	1,800
Equity method ownership percentage	70.00%
Number of buildings | building	3
Vornado Realty Trust
Real Estate Properties
Common limited partnership interest in the Operating Partnership	93.40%
Vornado Realty Trust | Alexanders Inc
Real Estate Properties
Equity method ownership percentage	32.40%