Organization and Business - (Narrative) (Details) ft² in Millions	12 Months Ended
	Dec. 31, 2021 ft² property unit building
Alexanders Inc
Real Estate Properties
Equity method ownership percentage	32.40%
New York | Manhattan
Real Estate Properties
Number of real estate properties | property	67
New York | Manhattan | Office Building
Real Estate Properties
Number of real estate properties | property	32
Square footage of real estate property (in sqft) | ft²	20.6
New York | Manhattan | Retail
Real Estate Properties
Number of real estate properties | property	60
Square footage of real estate property (in sqft) | ft²	2.7
New York | Manhattan | Residential Properties
Real Estate Properties
Number of real estate properties | property	8
Number of units in real estate property (in number of residential units) | unit	1,674
New York | Alexanders Inc | New York City Metropolitan Area
Real Estate Properties
Number of real estate properties | property	6
Equity method ownership percentage	32.40%
New York | Alexanders Inc | New York City Metropolitan Area | Residential Properties
Real Estate Properties
Number of units in real estate property (in number of residential units) | unit	312
New York | Alexanders Inc | New York City Metropolitan Area | Lexington Avenue 731
Real Estate Properties
Square footage of real estate property (in sqft) | ft²	1.1
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 (in sqft) | ft²	3.7
Other | San Francisco | 555 California Street | Office Building
Real Estate Properties
Square footage of real estate property (in sqft) | ft²	1.8
Equity method ownership percentage	70.00%
Number of buildings | building	3
Operating Partnership
Real Estate Properties
Common limited partnership interest in the Operating Partnership	92.60%