Schedule III - Real Estate and Accumulated Depreciation - Real Estate and Accumulated Depreciation, by Property (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Encumbrances, initial costs	$ 925,613
Land, initial costs	497,257
Building and improvements, initial costs	423,444
Real estate under development, initial costs	87,616
Cost capitalized subsequent to acquisition	107,882
Land, carrying amount	497,343
Building and improvements, carrying amount	493,389
Real estate under development, carrying amont	125,467
Total carrying amount	1,116,199	$ 1,070,606	$ 881,251	$ 770,779
Accumulated depreciation	90,462	$ 73,714	$ 58,174	$ 44,672
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	18,843
Land, carrying amount	273,103
Building and improvements, carrying amount	301,980
Real estate under development, carrying amont
Total carrying amount	575,083
Accumulated depreciation	33,088
Aspen [Member]
Encumbrances, initial costs	68,199
Land, initial costs	49,230
Building and improvements, initial costs	43,080
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	422
Land, carrying amount	49,230
Building and improvements, carrying amount	43,502
Real estate under development, carrying amont
Total carrying amount	92,732
Accumulated depreciation	2,714
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	36,505
Land, carrying amount	90,051
Building and improvements, carrying amount	86,026
Real estate under development, carrying amont
Total carrying amount	176,077
Accumulated depreciation	36,859
107 Columbia Heights in Brooklyn, NY [Member]
Encumbrances, initial costs	64,731
Land, initial costs
Building and improvements, initial costs
Real estate under development, initial costs	87,616
Cost capitalized subsequent to acquisition	37,851
Land, carrying amount
Building and improvements, carrying amount
Real estate under development, carrying amont	125,467
Total carrying amount	125,467
Accumulated depreciation
Property at 10 W 65th St. Manhattan, NY [Member]
Encumbrances, initial costs	34,350
Land, initial costs	63,677
Building and improvements, initial costs	15,337
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	407
Land, carrying amount	63,677
Building and improvements, carrying amount	15,744
Real estate under development, carrying amont
Total carrying amount	79,421
Accumulated depreciation	708
250 Livingston Street in Brooklyn [Member]
Encumbrances, initial costs	75,000
Land, initial costs	10,452
Building and improvements, initial costs	20,204
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	7,739
Land, carrying amount	10,452
Building and improvements, carrying amount	27,943
Real estate under development, carrying amont
Total carrying amount	38,395
Accumulated depreciation	11,168
141 Livingston Street in Brooklyn [Member]
Encumbrances, initial costs	77,333
Land, initial costs	10,830
Building and improvements, initial costs	12,079
Real estate under development, initial costs
Cost capitalized subsequent to acquisition	6,115
Land, carrying amount	10,830
Building and improvements, carrying amount	18,194
Real estate under development, carrying amont
Total carrying amount	29,024
Accumulated depreciation	$ 5,925