INVESTMENT IN UNCONSOLIDATED JOINT VENTURES (Narrative) (Details)		12 Months Ended
	May 09, 2012 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 02, 2014 ft²
Schedule of Equity Method Investments [Line Items]
Initial capital contribution		$ 12,620,000	$ 31,845,000	$ 1,320,000
Income from NIP		97,000	0	428,000
Investments in unconsolidated joint ventures		79,666,000	78,276,000
Equity in income of unconsolidated joint venture		1,260,000	6,621,000	(9,830,000)
110 William JV Partner
Schedule of Equity Method Investments [Line Items]
Income from NIP		0	5,200,000	0
NIP Joint Venture
Schedule of Equity Method Investments [Line Items]
Initial capital contribution	$ 8,000,000.0
Distributions		1,300,000	300,000	2,600,000
Income from NIP				400,000
Return of capital				$ 2,200,000
Investments in unconsolidated joint ventures		$ 0	1,225,000
NIP Joint Venture | Maximum
Schedule of Equity Method Investments [Line Items]
Ownership interest		5.00%
110 William Joint Venture
Schedule of Equity Method Investments [Line Items]
Ownership interest		60.00%			60.00%
Rentable square feet | ft²					928,157
Area of land | ft²					0.8
Investments in unconsolidated joint ventures		$ 0	0
Equity in income of unconsolidated joint venture			7,800,000
Equity in income (loss) of unconsolidated joint venture			$ 7,800,000
110 William Joint Venture | 110 William JV Partner
Schedule of Equity Method Investments [Line Items]
Ownership interest					40.00%