Organization (Details) ft² in Thousands, $ in Thousands		12 Months Ended
	Jun. 30, 2018 USD ($) ft²	Dec. 31, 2018 USD ($) ft² a property aptunits segment condomium_unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 04, 2017 ft²
Operating Properties
Number of properties | property		7
Property rentals | $		$ 152,795	$ 152,857	$ 151,444
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
Rego Park II | Tenant Occupant | Anchor | Toys R Us/ Babies R Us
Operating Properties
Area of property (in square feet)	47
Property rentals | $	$ 2,600
Rego Park II | Retail space | Toys R Us/ Babies R Us
Operating Properties
Area of property (in square feet)		47
Percentage of affiliate owned		33.33%
Operating property | 731 Lexington Avenue
Operating Properties
Area of property (in square feet)		1,311
Operating property | 731 Lexington Avenue | Office space
Operating Properties
Area of property (in square feet)		889
Operating property | 731 Lexington Avenue | Retail space
Operating Properties
Area of property (in square feet)		174
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 | Retail space | Tenant Occupant | Hennes Mauritz
Operating Properties
Area of property (in square feet)		27
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 | 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 | Alexander apartment tower
Operating Properties
Area of property (in square feet)		255
Number of property units | aptunits		312
Operating property | Paramus | Tenant Occupant | Ikea
Operating Properties
Area of land | a		30.3
Operating property | Flushing property
Operating Properties
Area of property (in square feet)		167
Properties to be developed | Rego Park III
Operating Properties
Area of land		140