Schedule III - Real Estate and Accumulated Depreciation - Real Estate and Accumulated Depreciation, by Property (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Encumbrances, initial costs	$ 1,009,431
Land, initial costs	540,773
Building and improvements, initial costs	467,544
Real estate under development, initial costs	31,129
Cost capitalized subsequent to acquisition	150,505
Land, carrying amount	540,859
Building and improvements, carrying amount	617,305
Real estate under development, carrying amont	31,787
Total carrying amount	1,189,951	$ 1,116,199	$ 1,070,606	$ 881,251
Accumulated depreciation	109,418	$ 90,462	$ 73,714	$ 58,174
Tribeca House Properties [Member]
Encumbrances, initial costs	360,000
Land, initial costs	273,103
Building and improvements, initial costs	283,137
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	23,634
Land, carrying amount	273,103
Building and improvements, carrying amount	306,771
Real estate under development, carrying amont
Total carrying amount	579,874
Accumulated depreciation	42,194
Aspen [Member]
Encumbrances, initial costs	66,862
Land, initial costs	49,230
Building and improvements, initial costs	43,080
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	1,633
Land, carrying amount	49,230
Building and improvements, carrying amount	44,713
Real estate under development, carrying amont
Total carrying amount	93,943
Accumulated depreciation	3,850
Flatbush Gardens, Brooklyn, NY [Member]
Encumbrances, initial costs	246,000
Land, initial costs	89,965
Building and improvements, initial costs	49,607
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	41,509
Land, carrying amount	90,051
Building and improvements, carrying amount	91,030
Real estate under development, carrying amont
Total carrying amount	181,081
Accumulated depreciation	41,706
Clover House [Member]
Encumbrances, initial costs	82,000
Land, initial costs	43,516
Building and improvements, initial costs	44,100
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	57,847
Land, carrying amount	43,516
Building and improvements, carrying amount	101,947
Real estate under development, carrying amont
Total carrying amount	145,463
Accumulated depreciation	693
Property at 10 W 65th St. Manhattan, NY [Member]
Encumbrances, initial costs	34,295
Land, initial costs	63,677
Building and improvements, initial costs	15,337
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	5,256
Land, carrying amount	63,677
Building and improvements, carrying amount	20,593
Real estate under development, carrying amont
Total carrying amount	84,270
Accumulated depreciation	1,492
Residential Property At 1010 Pacific Street [Member]
Encumbrances, initial costs	19,457
Land, initial costs
Building and improvements, initial costs
Real estate under development, initial costs	31,129
Cost capitalized subsequent to acquisition	658
Land, carrying amount
Building and improvements, carrying amount
Real estate under development, carrying amont	31,787
Total carrying amount	31,787
Accumulated depreciation
250 Livingston Street in Brooklyn [Member]
Encumbrances, initial costs	125,000
Land, initial costs	10,452
Building and improvements, initial costs	20,204
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	12,981
Land, carrying amount	10,452
Building and improvements, carrying amount	33,185
Real estate under development, carrying amont
Total carrying amount	43,637
Accumulated depreciation	12,508
141 Livingston Street in Brooklyn [Member]
Encumbrances, initial costs	75,817
Land, initial costs	10,830
Building and improvements, initial costs	12,079
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	6,987
Land, carrying amount	10,830
Building and improvements, carrying amount	19,066
Real estate under development, carrying amont
Total carrying amount	29,896
Accumulated depreciation	$ 6,975