Organization (Details)	12 Months Ended
	Dec. 31, 2016 a ft² Segment aptunits property	Dec. 31, 2001 a
Operating Properties
Number of properties in greater New York City metropolitan area (in property) | property	7
Number of Reportable Segments | Segment	1
Rego Park II | Retail space | Toys R Us/ Babies R Us
Operating Properties
Area of property (in square feet)	47,000
Paramus | Tenant Occupant | Ikea
Operating Properties
Area Of Land | a		30.3
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 | Office space | Tenant Occupant | Bloomberg
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 units in real estate property (in 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
Operating Properties
Area of property (in square feet)	609,000
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 | Rego Park II | Tenant Occupant | Anchor | Toys R Us/ Babies R Us
Operating Properties
Area of property (in square feet)	47,000
Operating property | Paramus
Operating Properties
Area Of Land | a	30.3
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
Operating property | Flushing property | Tenant Occupant | New World Mall LLC
Operating Properties
Area of property (in square feet)	167,000
Operating property | Alexander apartment tower
Operating Properties
Area of property (in square feet)	255,000
Number of units in real estate property (in units) | aptunits	312
Real Estate Property Percentage Leased	98.10%
Properties to be developed | Rego Park III
Operating Properties
Area Of Land	140,000