SEGMENT REPORTING - Segment Assets and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Segment Reporting Information [Line Items]
Property owned	$ 2,203,606	$ 1,812,557
Less accumulated depreciation	(426,926)	(399,249)
Total property owned	1,776,680	1,413,308
Mortgage loans receivable at fair value	42,160	24,661
Cash and cash equivalents	20,816	392	$ 16,804
Restricted cash	2,376	6,918	$ 2,199
Other assets	34,919	18,904
TOTAL ASSETS	1,876,951	1,464,183
Multifamily
Segment Reporting Information [Line Items]
Property owned	2,170,321	1,727,229
Less accumulated depreciation	(414,829)	(368,717)
Total property owned	1,755,492	1,358,512
All Other
Segment Reporting Information [Line Items]
Property owned	33,285	85,328
Less accumulated depreciation	(12,097)	(30,532)
Total property owned	$ 21,188	$ 54,796