INVESTMENT IN UNCONSOLIDATED JOINT VENTURES - Summarized Financial Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Real estate assets, net of accumulated depreciation and amortization	$ 822,498,000		$ 822,498,000		$ 759,479,000
Total assets	1,103,400,000		1,103,400,000		1,041,242,000
Other liabilities	19,745,000		19,745,000		19,801,000
Total liabilities, mezzanine equity and equity	1,103,400,000		1,103,400,000		1,041,242,000
Income Statement [Abstract]
Revenues	27,745,000	$ 25,282,000	75,869,000	$ 68,881,000
Real estate taxes and insurance	3,791,000	3,278,000	10,570,000	9,556,000
Depreciation and amortization	9,470,000	9,239,000	27,417,000	25,276,000
Interest expense	6,274,000	7,359,000	19,055,000	21,776,000
Total expenses	32,254,000	37,507,000	80,131,000	100,036,000
Net (loss) income attributable to common stockholders	(7,493,000)	772,000	(21,797,000)	11,304,000
Equity in income (loss) of unconsolidated entities	519,000	(419,000)	1,512,000	6,677,000
110 William Joint Venture
Balance Sheet Related Disclosures [Abstract]
Real estate assets, net of accumulated depreciation and amortization	247,304,000		247,304,000		242,430,000
Other assets	43,178,000		43,178,000		35,747,000
Total assets	290,482,000		290,482,000		278,177,000
Notes payable, net	312,664,000		312,664,000		292,221,000
Other liabilities	8,257,000		8,257,000		10,664,000
Partners’ deficit	(30,439,000)		(30,439,000)		(24,708,000)
Total liabilities, mezzanine equity and equity	290,482,000		290,482,000		278,177,000
Income Statement [Abstract]
Revenues	10,005,000	9,274,000	26,604,000	26,975,000
Real estate taxes and insurance	1,935,000	1,818,000	5,580,000	5,248,000
Depreciation and amortization	3,139,000	2,946,000	8,721,000	8,449,000
Interest expense	4,125,000	4,084,000	12,093,000	12,816,000
Total expenses	11,295,000	11,215,000	32,388,000	33,331,000
Total other income	12,000	34,000	50,000	105,000
Net (loss) income attributable to common stockholders	(1,278,000)	(1,907,000)	(5,734,000)	(6,251,000)
Company’s share of net loss	(767,000)	(1,144,000)	(3,440,000)	(3,751,000)
Loss from equity method investments	0	0	0	300,000
110 William Joint Venture | Operating, maintenance, and management costs
Income Statement [Abstract]
Expenses	2,096,000	2,367,000	5,994,000	6,818,000
353 Sacramento Joint Venture
Balance Sheet Related Disclosures [Abstract]
Real estate assets, net of accumulated depreciation and amortization	183,176,000		183,176,000		180,592,000
Other assets	18,434,000		18,434,000		21,822,000
Total assets	201,610,000		201,610,000		202,414,000
Notes payable, net	115,500,000		115,500,000		115,280,000
Other liabilities	7,507,000		7,507,000		11,193,000
Partners’ deficit	78,603,000		78,603,000		75,941,000
Total liabilities, mezzanine equity and equity	201,610,000		201,610,000		$ 202,414,000
Income Statement [Abstract]
Revenues	5,520,000	3,933,000	15,771,000	12,278,000
Real estate taxes and insurance	755,000	700,000	2,235,000	2,095,000
Depreciation and amortization	1,894,000	1,718,000	5,135,000	4,849,000
Interest expense	948,000	1,420,000	3,290,000	4,286,000
Total expenses	4,529,000	4,761,000	13,110,000	13,923,000
Net (loss) income attributable to common stockholders	991,000	(828,000)	2,661,000	(1,645,000)
Equity in income (loss) of unconsolidated entities	583,000	(419,000)	1,576,000	(798,000)
353 Sacramento Joint Venture | Operating, maintenance, and management costs
Income Statement [Abstract]
Expenses	$ 932,000	$ 923,000	$ 2,450,000	$ 2,693,000