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RELATED PARTY TRANSACTIONS - Related-party Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Payable as of	$ 6,830		$ 6,830		$ 5,552
Advisor and Dealer Manager
Related Party Transaction [Line Items]
Incurred	544	$ 572	2,008	$ 1,673
Payable as of	6,830		6,830		5,552
Advisor and Dealer Manager \| Asset management fees
Related Party Transaction [Line Items]
Expenses	432	483	1,293	1,412
Payable as of	5,464		5,464		4,171
Advisor and Dealer Manager \| Reimbursement of operating expenses
Related Party Transaction [Line Items]
Expenses	80	49	232	139
Payable as of	25		25		29
Advisor and Dealer Manager \| Property management fees
Related Party Transaction [Line Items]
Expenses	32	40	102	122
Payable as of	3		3		14
Advisor and Dealer Manager \| Disposition fees
Related Party Transaction [Line Items]
Expenses	0	0	381	0
Payable as of	0		0		0
Advisor and Dealer Manager \| Advisor advance for cash distributions
Related Party Transaction [Line Items]
Incurred	0	$ 0	0	$ 0
Payable as of	$ 1,338		$ 1,338		$ 1,338
KBS Capital Advisors LLC \| Cost of Investments
Related Party Transaction [Line Items]
Property management fee, percent fee			1.00%