Organization (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019 a ft² property segment unit condomium_unit	Dec. 31, 2017 USD ($)	Sep. 23, 2019 ft²	Apr. 04, 2017 ft²
Operating Properties
Number of properties | property	7
Number of segments | segment	1
Rego Park I | Sears
Operating Properties
Area of property (in square feet)				195
Property rentals | $		$ 10,300
Rego Park II | Kohl's
Operating Properties
Lease expiration date	Jan. 01, 2031
Rego Park II | Anchor
Operating Properties
Area of property (in square feet)	609
Operating property | 731 Lexington Avenue
Operating Properties
Area of property (in square feet)	1,323
Operating property | 731 Lexington Avenue | Office space
Operating Properties
Area of property (in square feet)	920
Operating property | 731 Lexington Avenue | Retail space
Operating Properties
Area of property (in square feet)	155
Operating property | 731 Lexington Avenue | Retail space | Tenant Occupant | Home Depot
Operating Properties
Area of property (in square feet)	83
Operating property | 731 Lexington Avenue | Retail space | Tenant Occupant | Container Store
Operating Properties
Area of property (in square feet)	34
Operating property | 731 Lexington Avenue | Residence space
Operating Properties
Area of property (in square feet)	248
Number of property units | condomium_unit	105
Operating property | Rego Park I
Operating Properties
Area of property (in square feet)	343
Operating property | Rego Park I | Tenant Occupant | Anchor | Sears
Operating Properties
Area of property (in square feet)	195
Operating property | Rego Park I | Tenant Occupant | Anchor | IKEA
Operating Properties
Area of property (in square feet)			113
Operating property | Rego Park I | Tenant Occupant | Anchor | Burlington Coat Factory
Operating Properties
Area of property (in square feet)	50
Operating property | Rego Park I | Tenant Occupant | Anchor | Bed Bath and Beyond
Operating Properties
Area of property (in square feet)	46
Operating property | Rego Park I | Tenant Occupant | Anchor | Marshalls
Operating Properties
Area of property (in square feet)	36
Operating property | Rego Park II | Tenant Occupant | Anchor | Costco
Operating Properties
Area of property (in square feet)	145
Operating property | Rego Park II | Tenant Occupant | Anchor | Century 21
Operating Properties
Area of property (in square feet)	135
Operating property | Rego Park II | Tenant Occupant | Anchor | Kohl's
Operating Properties
Area of property (in square feet)	133
Operating property | The Alexander apartment tower
Operating Properties
Area of property (in square feet)	255
Number of property units | unit	312
Operating property | Paramus, NJ | Tenant Occupant | IKEA
Operating Properties
Area of land | a	30.3
Operating property | Flushing
Operating Properties
Area of property (in square feet)	167
Properties to be developed | Rego Park III
Operating Properties
Area of land	140