Organization (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² a property aptunits segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 04, 2017 ft²
Operating Properties
Number of properties | property	7
Property rentals | $	$ 152,857	$ 151,444	$ 138,688
Number of segments | segment	1
Rego Park I | Sears
Operating Properties
Area of property (in square feet)				195,000
Maturity date	Mar. 01, 2020
Property rentals | $	$ 10,600
Rego Park II | Anchor
Operating Properties
Area of property (in square feet)	609,000
Rego Park II | Retail space | Toys R Us/ Babies R Us
Operating Properties
Area of property (in square feet)	47,000
Operating property | 731 Lexington Avenue
Operating Properties
Area of property (in square feet)	1,311,000
Operating property | 731 Lexington Avenue | Office space
Operating Properties
Area of property (in square feet)	889,000
Operating property | 731 Lexington Avenue | Retail space
Operating Properties
Area of property (in square feet)	174,000
Operating property | 731 Lexington Avenue | Retail space | Tenant Occupant | Home Depot
Operating Properties
Area of property (in square feet)	83,000
Operating property | 731 Lexington Avenue | Retail space | Tenant Occupant | Container Store
Operating Properties
Area of property (in square feet)	34,000
Operating property | 731 Lexington Avenue | Retail space | Tenant Occupant | Hennes Mauritz
Operating Properties
Area of property (in square feet)	27,000
Operating property | 731 Lexington Avenue | Residence space
Operating Properties
Area of property (in square feet)	248,000
Number of property units | aptunits	105
Operating property | Rego Park I
Operating Properties
Area of property (in square feet)	343,000
Operating property | Rego Park I | Tenant Occupant | Anchor | Sears
Operating Properties
Area of property (in square feet)	195,000
Operating property | Rego Park I | Tenant Occupant | Anchor | Burlington Coat Factory
Operating Properties
Area of property (in square feet)	50,000
Operating property | Rego Park I | Tenant Occupant | Anchor | Bed Bath and Beyond
Operating Properties
Area of property (in square feet)	46,000
Operating property | Rego Park I | Tenant Occupant | Anchor | Marshalls
Operating Properties
Area of property (in square feet)	36,000
Operating property | Rego Park II | Tenant Occupant | Anchor | Costco
Operating Properties
Area of property (in square feet)	145,000
Operating property | Rego Park II | Tenant Occupant | Anchor | Century 21
Operating Properties
Area of property (in square feet)	135,000
Operating property | Rego Park II | Tenant Occupant | Anchor | Kohl's
Operating Properties
Area of property (in square feet)	133,000
Operating property | Alexander apartment tower
Operating Properties
Area of property (in square feet)	255,000
Number of property units | aptunits	312
Real estate property percentage leased	94.60%
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)	167,000
Properties to be developed | Rego Park III
Operating Properties
Area of land	140,000