Leases (Narratives) (Details)	1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2016 USD ($) ft²	Dec. 31, 2016 USD ($) options	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) options	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
As Lessor
Additional Rent Based on Percentage of Tenant's sales										$ 182,000	$ 94,000	$ 108,000
Real estate revenue, net (in dollars)		$ 57,253,000	$ 57,120,000	$ 57,005,000	$ 55,558,000	$ 52,819,000	$ 52,414,000	$ 50,646,000	$ 52,036,000	226,936,000	207,915,000	200,814,000
Leasing Fees | Vornado
As Lessor
Fees paid (in US dollars)		$ 415,000				$ 2,473,000				$ 415,000	2,473,000
Minimum
Leases Lease Terms
Lease term range as Lessor										5 years
Maximum
Leases Lease Terms
Lease term range as Lessor										25 years
Flushing property
As Lessee
Rent expense										$ 746,000	746,000	746,000
Flushing property | Ten year extension option
As Lessee
Lease expiration year										2027
Number Of Extension Options | options		1								1
Lease extension period										10 years
731 Lexington Avenue | Office space | Bloomberg | Initial Lease Agreement
As Lessor
Area of property (in square feet) | ft²	697,000
As Lessee
Leases Expiration Date	Feb. 28, 2029
Lease extension period	10 years
731 Lexington Avenue | Office space | Bloomberg | Lease Amendment
As Lessor
Area of property (in square feet) | ft²	192,000
Letter of credit issued by customer	$ 200,000,000
As Lessee
Leases Expiration Date	Feb. 28, 2029
Lease extension period	10 years
731 Lexington Avenue | Office space | Bloomberg | Lease Amendment | Leasing Fees
As Lessor
Fees paid (in US dollars)	$ 8,916,000
731 Lexington Avenue | Office space | Bloomberg | Lease Amendment | Leasing Fees | Third party Broker
As Lessor
Fees paid (in US dollars)	7,200,000
731 Lexington Avenue | Office space | Bloomberg | Lease Amendment | Leasing Fees | Vornado
As Lessor
Fees paid (in US dollars)	$ 1,716,000
Alexander apartment tower | Residence space | Minimum
Leases Lease Terms
Lease term range as Lessor										1 year
Alexander apartment tower | Residence space | Maximum
Leases Lease Terms
Lease term range as Lessor										2 years
Customer Concentration Risk | Bloomberg
As Lessor
Percentage Of Minimum Revenue Threshold Contributed By One Tenant (in percentage)										10.00%
Customer Concentration Risk | Bloomberg | Sales Revenue Services Net
As Lessor
Real estate revenue, net (in dollars)										$ 104,590,000	$ 94,468,000	$ 91,109,000
Percentage Rent Contributed By Tenant										46.00%	45.00%	45.00%