Schedule III: Real estate and Accumulated depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 1,164,544
Initial cost of Land	21,593
Initial cost of Building and Leasehold improvements	26,239
Costs capitalized subsequent to acquisition	1,023,211
Carrying amount of Land	44,971
Carrying amount of Buildings and Leasehold improvements	1,014,311
Development and Construction in progress	11,761
Total	1,071,043	$ 1,041,342	$ 1,027,691	$ 1,037,368
Accumulated Depreciation and Amortization	350,122	324,499	$ 297,421	$ 283,044
Deferred debt issuance costs, net	8,374
Differences between book and tax basis	139,364
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	84,261
Carrying amount of Land	1,647
Carrying amount of Buildings and Leasehold improvements	86,721
Development and Construction in progress	6,493
Total	94,861
Accumulated Depreciation and Amortization	$ 38,451
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
Life on which Depreciation in Latest Income Statement is Computed	3 years
New York | Rego Park I | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	39 years
New York | Rego Park II | Retail space
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 202,544
Initial cost of Land	3,127
Initial cost of Building and Leasehold improvements	1,467
Costs capitalized subsequent to acquisition	389,150
Carrying amount of Land	3,127
Carrying amount of Buildings and Leasehold improvements	390,114
Development and Construction in progress	503
Total	393,744
Accumulated Depreciation and Amortization	$ 113,318
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
Life on which Depreciation in Latest Income Statement is Computed	3 years
New York | Rego Park II | Retail space | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	40 years
New York | The Alexander apartment tower
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Encumbrances	$ 94,000
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	$ 21,028
Date of construction	2016
Date acquired	1992
New York | The Alexander apartment tower | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	3 years
New York | The Alexander apartment tower | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	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	5,292
Carrying amount of Land	779
Carrying amount of Buildings and Leasehold improvements	527
Development and Construction in progress	4,765
Total	6,071
Accumulated Depreciation and Amortization	$ 325
Date acquired	1992
New York | Rego Park III | Minimum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	5 years
New York | Rego Park III | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	15 years
New York | Flushing
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	$ 1,146
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	$ 800,000
Initial cost of Land	14,432
Initial cost of Building and Leasehold improvements	12,355
Costs capitalized subsequent to acquisition	416,994
Carrying amount of Land	27,497
Carrying amount of Buildings and Leasehold improvements	416,284
Development and Construction in progress	0
Total	443,781
Accumulated Depreciation and Amortization	$ 175,854
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
Life on which Depreciation in Latest Income Statement is Computed	9 years
New York | Lexington Avenue | Maximum
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation
Life on which Depreciation in Latest Income Statement is Computed	39 years
New Jersey | Paramus, NJ
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	$ 50,000	$ 195,708