RELATED PARTY TRANSACTIONS (Narrative1) (Details)			12 Months Ended			121 Months Ended
	Oct. 29, 2019	Mar. 20, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) unit shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) unit shares	May 12, 2017 USD ($)	Oct. 28, 2016 USD ($)
Related Party Transaction [Line Items]
Redemptions of common stock		$ 8,600,000	$ 10,028,000	$ 123,613,000	$ 74,780,000	$ 285,400,000
Battery Point
Related Party Transaction [Line Items]
Number of securities received (in shares) | shares		210,000
Price per share (in dollars per share) | $ / shares		$ 25
Stock repurchased during period (in shares) | shares			430,000
Stock repurchased during period			$ 10,800,000
Series B Preferred Units | Battery Point
Related Party Transaction [Line Items]
Redemptions of common stock (in shares) | shares		13,000
Stock redeemed (in dollars per share) | $ / shares		$ 1,000
Principal paydow		$ 900,000
Series A-3 Preferred Units | Battery Point
Related Party Transaction [Line Items]
Investments		$ 5,300,000
Redemptions of common stock (in shares) | shares		13,000
Stock redeemed (in dollars per share) | $ / shares		$ 1,000
Principal paydow		$ 7,700,000
Number of securities received (in shares) | shares		210,000
Price per share (in dollars per share) | $ / shares		$ 25
Battery Point
Related Party Transaction [Line Items]
Redemptions of common stock (in shares) | shares		13,000
Stock redeemed (in dollars per share) | $ / shares		$ 1,000
Redemptions of common stock		$ 8,600,000
Number of securities received (in shares) | shares		210,000
Price per share (in dollars per share) | $ / shares		$ 25
Battery Point | Series B Preferred Units
Related Party Transaction [Line Items]
Investments to be made							$ 17,500,000
Investments				4,500,000
Dividend rate, percentage	12.00%
Battery Point | Series B Preferred Units | Maximum
Related Party Transaction [Line Items]
Investments to be made								$ 25,000,000
Battery Point | Series A-3 Preferred Units
Related Party Transaction [Line Items]
Number of securities received (in shares) | shares			640,000			640,000
Stock repurchased during period (in shares) | shares			430,000
Stock repurchased during period			$ 10,800,000
Pacific Oak Opportunity Zone Fund I
Related Party Transaction [Line Items]
Investments			$ 20,600,000			$ 20,600,000
Acquisition fee, percent of purchase price fee			1.50%
Number of units in real estate property | unit			91			91
Investment, purchase price, benchmark			$ 25,000,000.0
Acquisition fee of purchase price fee in excess of benchmark purchase price			1.00%
Asset management fee, percent			0.25%			0.25%
Financing fee as percent of original principal amount of any indebtedness			0.50%			0.50%
KBS Capital Advisors LLC
Related Party Transaction [Line Items]
Incurred			$ 44,893,000	$ 14,762,000	$ 20,615,000
KBS Capital Advisors LLC | Subordinated Participation in Net Cash Flows
Related Party Transaction [Line Items]
Noncompounded return on invested capital as percent per year, percent			7.00%			7.00%
Percent of net cash flows to be received by related party, percent			15.00%			15.00%
Distributions paid from operating cash flows, annual return, percent			7.00%			7.00%
KBS Capital Advisors LLC | Subordinated Incentive Listing Fee
Related Party Transaction [Line Items]
Noncompounded return on invested capital as percent per year, percent			7.00%			7.00%
Percent of net cash flows to be received by related party, percent			15.00%			15.00%
Distributions paid from operating cash flows, annual return, percent			7.00%			7.00%
KBS Capital Advisors LLC | Subordinated Performance Fee Due Upon Termination
Related Party Transaction [Line Items]
Noncompounded return on invested capital as percent per year, percent			7.00%			7.00%
Percent of net cash flows to be received by related party, percent			15.00%			15.00%
Distributions paid from operating cash flows, annual return, percent			7.00%			7.00%
Battery Point Holdings | DayMark
Related Party Transaction [Line Items]
Noncontrolling interest		40.00%
DayMark Service Entities
Related Party Transaction [Line Items]
Property management fee, percent of gross asset value fee		1.50%
Annual property management fee, percent of tenants rent received fee		8.00%
Acquisition fee, percent of purchase price fee		1.00%
Commission fee from sales as broker		1.00%