RELATED PARTY TRANSACTIONS - Additional Information (Details)		3 Months Ended		9 Months Ended
	Mar. 14, 2019 USD ($) $ / ft²	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	May 29, 2015 ft²
Related Party Transaction [Line Items]
Non-compounded return on net invested capital		8.00%		8.00%
Accrued amount		$ 9,012,000		$ 9,012,000		$ 8,626,000
Annualized base rent		66,584,000		66,584,000
Affiliated Entity | Subsidiaries
Related Party Transaction [Line Items]
Net rentable area (in sq feet) | ft²							5,046
Percent of total rentable square feet							2.40%
Annualized base rent	$ 300,000
Average annualized base rent per square foot | $ / ft²	62.55
Rental income		81,000	$ 80,000	245,000	$ 241,000
Asset management fees
Related Party Transaction [Line Items]
Accrued amount		8,800,000		8,800,000		8,500,000
Deferred amount		$ 8,000,000		8,000,000		$ 7,200,000
KBS Capital Advisors LLC
Related Party Transaction [Line Items]
Incurred				$ 79,000	$ 74,000