Schedule III: Real estate and Accumulated depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 1,056,147
Initial cost of Land	21,593
Initial cost of Building, Leaseholds and Leasehold improvements	26,239
Costs capitalized subsequent to acquisition	985,719
Carrying amount of Land	44,971
Carring amount of Buildings and Leasehold improvements	985,800
Development and Construction in progress	2,780
Total	1,033,551	$ 1,029,472	$ 993,927	$ 919,576
Accumulated depreciation and amortization	252,737	$ 225,533	$ 210,025	$ 185,375
Deferred debt issuance costs, net	3,788
Differences Between Book and Tax Basis	208,622
All other properties
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	0
Initial cost of Land	167
Initial cost of Building, Leaseholds and Leasehold improvements	1,804
Costs capitalized subsequent to acquisition	(1,804)
Carrying amount of Land	167
Carring amount of Buildings and Leasehold improvements	0
Development and Construction in progress	0
Total	167
Accumulated depreciation and amortization	$ 0
Date acquired	1992
New York | Rego Park I
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 78,246
Initial cost of Land	1,647
Initial cost of Building, Leaseholds and Leasehold improvements	8,953
Costs capitalized subsequent to acquisition	52,368
Carrying amount of Land	1,647
Carring amount of Buildings and Leasehold improvements	61,285
Development and Construction in progress	36
Total	62,968
Accumulated depreciation and amortization	$ 31,212
Date of construction	1959
Date acquired	1992
New York | Rego Park I | Minimum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	3 years
New York | Rego Park I | Maximum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	39 years
New York | Rego Park II | Retail space
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 259,901
Initial cost of Land	3,127
Initial cost of Building, Leaseholds and Leasehold improvements	1,467
Costs capitalized subsequent to acquisition	386,655
Carrying amount of Land	3,127
Carring amount of Buildings and Leasehold improvements	387,999
Development and Construction in progress	123
Total	391,249
Accumulated depreciation and amortization	$ 73,186
Date of construction	2009
Date acquired	1992
New York | Rego Park II | Retail space | Minimum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	3 years
New York | Rego Park II | Retail space | Maximum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	40 years
New York | Alexander apartment tower | Residence space
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 0
Initial cost of Land	0
Initial cost of Building, Leaseholds and Leasehold improvements	0
Costs capitalized subsequent to acquisition	119,105
Carrying amount of Land	0
Carring amount of Buildings and Leasehold improvements	119,105
Development and Construction in progress	0
Total	119,105
Accumulated depreciation and amortization	$ 6,081
Date of construction	2016
Date acquired	1992
New York | Alexander apartment tower | Residence space | Minimum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	3 years
New York | Alexander apartment tower | Residence space | Maximum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	39 years
New York | Rego Park III | Retail space
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 0
Initial cost of Land	779
Initial cost of Building, Leaseholds and Leasehold improvements	0
Costs capitalized subsequent to acquisition	3,124
Carrying amount of Land	779
Carring amount of Buildings and Leasehold improvements	503
Development and Construction in progress	2,621
Total	3,903
Accumulated depreciation and amortization	$ 198
Date acquired	1992
New York | Rego Park III | Retail space | Minimum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	5 years
New York | Rego Park III | Retail space | Maximum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	15 years
New York | Flushing property
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 0
Initial cost of Land	0
Initial cost of Building, Leaseholds and Leasehold improvements	1,660
Costs capitalized subsequent to acquisition	(107)
Carrying amount of Land	0
Carring amount of Buildings and Leasehold improvements	1,553
Development and Construction in progress	0
Total	1,553
Accumulated depreciation and amortization	$ 909
Date of construction	1975
Date acquired	1992
New York | Lexington Avenue
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 650,000
Initial cost of Land	14,432
Initial cost of Building, Leaseholds and Leasehold improvements	12,355
Costs capitalized subsequent to acquisition	416,065
Carrying amount of Land	27,497
Carring amount of Buildings and Leasehold improvements	415,355
Development and Construction in progress	0
Total	442,852
Accumulated depreciation and amortization	$ 141,151
Date of construction	2003
Date acquired	1992
New York | Lexington Avenue | Minimum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	9 years
New York | Lexington Avenue | Maximum
Reconciliation of carrying amount of real estate investments rollforward
Property, Plant and Equipment, Useful Life	39 years
New Jersey | Paramus
Reconciliation of carrying amount of real estate investments rollforward
Encumbrances	$ 68,000
Initial cost of Land	1,441
Initial cost of Building, Leaseholds and Leasehold improvements	0
Costs capitalized subsequent to acquisition	10,313
Carrying amount of Land	11,754
Carring amount of Buildings and Leasehold improvements	0
Development and Construction in progress	0
Total	11,754
Accumulated depreciation and amortization	$ 0
Date acquired	1992