Schedule III: Real estate and Accumulated depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 1,170,544
Initial cost of Land	21,593
Initial cost of Building and Leasehold improvements	26,239
Costs capitalized subsequent to acquisition	979,859
Carrying amount of Land	44,971
Carrying amount of Buildings and Leasehold improvements	978,474
Development and Construction in progress	4,246
Total	1,027,691	$ 1,037,368	$ 1,033,551	$ 1,029,472
Accumulated Depreciation and Amortization	297,421	283,044	$ 252,737	$ 225,533
Deferred debt issuance costs, net	9,010
Differences between book and tax basis	186,559
Other Properties
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost of Land	167
Initial cost of Building 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
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 0
Initial cost of Land	1,647
Initial cost of Building and Leasehold improvements	8,953
Costs capitalized subsequent to acquisition	54,456
Carrying amount of Land	1,647
Carrying amount of Buildings and Leasehold improvements	62,836
Development and Construction in progress	573
Total	65,056
Accumulated Depreciation and Amortization	$ 34,987
Date of construction	1959
Date acquired	1992
New York | Rego Park I | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	3 years
New York | Rego Park I | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	39 years
New York | Rego Park II | Retail space
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 252,544
Initial cost of Land	3,127
Initial cost of Building and Leasehold improvements	1,467
Costs capitalized subsequent to acquisition	387,884
Carrying amount of Land	3,127
Carrying amount of Buildings and Leasehold improvements	389,191
Development and Construction in progress	160
Total	392,478
Accumulated Depreciation and Amortization	$ 93,011
Date of construction	2009
Date acquired	1992
New York | Rego Park II | Retail space | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	3 years
New York | Rego Park II | Retail space | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	40 years
New York | Alexander apartment tower
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 0
Initial cost of Land	0
Initial cost of Building 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	$ 13,631
Date of construction	2016
Date acquired	1992
New York | Alexander apartment tower | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	3 years
New York | Alexander apartment tower | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	39 years
New York | Rego Park III
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 0
Initial cost of Land	779
Initial cost of Building and Leasehold improvements	0
Costs capitalized subsequent to acquisition	4,016
Carrying amount of Land	779
Carrying amount of Buildings and Leasehold improvements	503
Development and Construction in progress	3,513
Total	4,795
Accumulated Depreciation and Amortization	$ 259
Date acquired	1992
New York | Rego Park III | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	5 years
New York | Rego Park III | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	15 years
New York | Flushing property
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 0
Initial cost of Land	0
Initial cost of Building 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	$ 1,028
Date of construction	1975
Date acquired	1992
New York | Lexington Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 850,000
Initial cost of Land	14,432
Initial cost of Building and Leasehold improvements	12,355
Costs capitalized subsequent to acquisition	405,989
Carrying amount of Land	27,497
Carrying amount of Buildings and Leasehold improvements	405,279
Development and Construction in progress	0
Total	432,776
Accumulated Depreciation and Amortization	$ 154,505
Date of construction	2003
Date acquired	1992
New York | Lexington Avenue | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	9 years
New York | Lexington Avenue | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Property, Plant and Equipment, Useful Life	39 years
New Jersey | Paramus
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 68,000
Initial cost of Land	1,441
Initial cost of Building 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
Participation Agreement | Rego Park II
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Loan participation balance	$ 195,708	$ 198,537