Schedule III: Real estate and Accumulated depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
REAL ESTATE:
Encumbrances	$ 1,252,440
Initial cost of Land	21,593
Initial cost of Building, Leaseholds and Leasehold improvements	26,239
Costs capitalized subsequent to acquisition	989,536
Carrying amount of Land	44,971
Carrying amount of Buildings and Leasehold improvements	988,846
Development and Construction in progress	3,551
Total	1,037,368	$ 1,033,551	$ 1,029,472	$ 993,927
Accumulated Depreciation and Amortization	283,044	$ 252,737	$ 225,533	$ 210,025
Deferred debt issuance costs, net	12,218
Differences between book and tax basis	199,268
Other Properties
REAL ESTATE:
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
Carrying 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
REAL ESTATE:
Encumbrances	$ 78,246
Initial cost of Land	1,647
Initial cost of Building, Leaseholds and Leasehold improvements	8,953
Costs capitalized subsequent to acquisition	53,390
Carrying amount of Land	1,647
Carrying amount of Buildings and Leasehold improvements	62,268
Development and Construction in progress	75
Total	63,990
Accumulated Depreciation and Amortization	$ 33,073
Date of construction	1959
Date acquired	1992
New York | Rego Park I | Minimum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	3 years
New York | Rego Park I | Maximum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	39 years
New York | Rego Park II | Retail space
REAL ESTATE:
Encumbrances	$ 256,194
Initial cost of Land	3,127
Initial cost of Building, Leaseholds and Leasehold improvements	1,467
Costs capitalized subsequent to acquisition	388,890
Carrying amount of Land	3,127
Carrying amount of Buildings and Leasehold improvements	390,118
Development and Construction in progress	239
Total	393,484
Accumulated Depreciation and Amortization	$ 83,963
Date of construction	2009
Date acquired	1992
New York | Rego Park II | Retail space | Minimum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	3 years
New York | Rego Park II | Retail space | Maximum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	40 years
New York | Alexander apartment tower
REAL ESTATE:
Encumbrances	$ 0
Initial cost of Land	0
Initial cost of Building, Leaseholds and Leasehold improvements	0
Costs capitalized subsequent to acquisition	119,112
Carrying amount of Land	0
Carrying amount of Buildings and Leasehold improvements	119,112
Development and Construction in progress	0
Total	119,112
Accumulated Depreciation and Amortization	$ 9,856
Date of construction	2016
Date acquired	1992
New York | Alexander apartment tower | Minimum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	3 years
New York | Alexander apartment tower | Maximum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	39 years
New York | Rego Park III
REAL ESTATE:
Encumbrances	$ 0
Initial cost of Land	779
Initial cost of Building, Leaseholds and Leasehold improvements	0
Costs capitalized subsequent to acquisition	3,740
Carrying amount of Land	779
Carrying amount of Buildings and Leasehold improvements	503
Development and Construction in progress	3,237
Total	4,519
Accumulated Depreciation and Amortization	$ 228
Date acquired	1992
New York | Rego Park III | Minimum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	5 years
New York | Rego Park III | Maximum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	15 years
New York | Flushing property
REAL ESTATE:
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
Carrying amount of Buildings and Leasehold improvements	1,553
Development and Construction in progress	0
Total	1,553
Accumulated Depreciation and Amortization	$ 968
Date of construction	1975
Date acquired	1992
New York | Lexington Avenue
REAL ESTATE:
Encumbrances	$ 850,000
Initial cost of Land	14,432
Initial cost of Building, Leaseholds and Leasehold improvements	12,355
Costs capitalized subsequent to acquisition	416,002
Carrying amount of Land	27,497
Carrying amount of Buildings and Leasehold improvements	415,292
Development and Construction in progress	0
Total	442,789
Accumulated Depreciation and Amortization	$ 154,956
Date of construction	2003
Date acquired	1992
New York | Lexington Avenue | Minimum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	9 years
New York | Lexington Avenue | Maximum
REAL ESTATE:
Property, Plant and Equipment, Useful Life	39 years
New Jersey | Paramus
REAL ESTATE:
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
Carrying amount of Buildings and Leasehold improvements	0
Development and Construction in progress	0
Total	11,754
Accumulated Depreciation and Amortization	$ 0
Date acquired	1992