RELATED PARTY TRANSACTIONS - Additional Information (Details)							3 Months Ended				9 Months Ended		12 Months Ended	130 Months Ended
	Aug. 31, 2020 USD ($)	Jul. 01, 2020 USD ($) shares	Mar. 27, 2020 shares	Nov. 01, 2019	Oct. 29, 2019	Mar. 20, 2019 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) unit shares	Sep. 30, 2020 USD ($)	Dec. 31, 2018 USD ($)	May 12, 2017 USD ($)	Oct. 28, 2016 USD ($)
Related Party Transaction [Line Items]
Redemptions of common stock						$ 8,600,000	$ 6,000			$ 2,326,000	$ 830,000	$ 7,433,000		$ 286,300,000
Investments in unconsolidated entities							$ 69,686,000				$ 69,686,000		$ 78,276,000	$ 69,686,000
Battery Point
Related Party Transaction [Line Items]
Shares acquired during period (in shares) | shares		1,000,000
Battery Point | PORT OP LP | BPT Holdings, LLC
Related Party Transaction [Line Items]
Common equity units received in transaction (in units) | shares		510,816
Percent of outstanding common equity units received in transaction		4.50%
Common equity units received in transaction		$ 3,000,000.0
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%
KBS Capital Advisors LLC | Subordinated Performance Fee Due Upon Termination
Related Party Transaction [Line Items]
Issuance of restricted stock (in shares) | shares			3,411,737
Period of redemption after vesting											60 days
Termination fee as percent of liquidation proceeds, percent							50.00%				50.00%			50.00%
Percent of total operating expenses in four consecutive quarters							2.00%				2.00%			2.00%
Percent of total operating expenses in four consecutive quarters, benchmark							25.00%				25.00%			25.00%
KBS Capital Advisors LLC | Subordinated Performance Fee Due Upon Termination | After November 1, 2024
Related Party Transaction [Line Items]
Period of redemption of remaining shares											60 days
DayMark | Battery Point Holdings
Related Party Transaction [Line Items]
Noncontrolling interest						40.00%
Point Trust LLC
Related Party Transaction [Line Items]
Investment acquired (in shares) | shares						210,000
Investment acquired (in dollars per shares) | $ / shares						$ 25
Series B Preferred Units | Point Trust LLC
Related Party Transaction [Line Items]
Redemptions of common stock (in shares) | shares						13,000
Redemptions of common stock (in dollars per shares) | $ / shares						$ 1,000
Principal paydown						$ 900,000
Series A-3 Preferred Units | Point Trust LLC
Related Party Transaction [Line Items]
Shares eliminated in consolidation due to acquisition | shares		640,000
Point Trust LLC
Related Party Transaction [Line Items]
Investment acquired (in shares) | shares						210,000
Investment acquired (in dollars per shares) | $ / shares						$ 25
Number of shares purchased (in shares) | shares													430,000
Stock repurchased during period													$ 10,800,000
Point Trust LLC | 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%
Redemptions of common stock (in shares) | shares						13,000
Redemptions of common stock (in dollars per shares) | $ / shares						$ 1,000
Point Trust LLC | Series B Preferred Units | Maximum
Related Party Transaction [Line Items]
Investments to be made																	$ 25,000,000
Point Trust LLC | Series A-3 Preferred Units
Related Party Transaction [Line Items]
Investments						$ 5,300,000
Principal paydown						$ 7,700,000
Number of shares purchased (in shares) | shares									430,000
Stock repurchased during period									$ 10,800,000
Shares eliminated in consolidation due to acquisition | shares		640,000
Pacific Oak Opportunity Zone Fund I
Related Party Transaction [Line Items]
Investments							$ 20,900,000				$ 20,900,000		20,600,000	$ 20,900,000
Acquisition fee, percent of purchase price fee									1.50%
Investments in unconsolidated entities							20,891,000				20,891,000		$ 20,846,000	20,891,000
Number of units in real estate property | unit													91
Acquisition fee of purchase price fee in excess of benchmark purchase price									1.00%
Investment, purchase price, benchmark									$ 25,000,000.0
Asset management fee, percent													0.25%
Financing fee as percent of original principal amount of any indebtedness													0.50%
Pacific Oak Opportunity Zone Fund I | Asset management fees
Related Party Transaction [Line Items]
Waived asset management fees											500,000
Due from related parties							100,000				100,000			100,000
Battery Point Series A-3 Preferred Units
Related Party Transaction [Line Items]
Investments in unconsolidated entities							$ 0				0		$ 13,991,000	$ 0
Battery Point Series A-3 Preferred Units | Series A-3 Preferred Units
Related Party Transaction [Line Items]
Investments in unconsolidated entities								$ 14,000,000.0
Gains on investment prior to acquisition								$ 2,000,000.0
PORT II OP LP
Related Party Transaction [Line Items]
Payments to acquire investments											5,000,000.0
Shared fees as percent of application fee collected	100.00%
Shared fees as percent of other fees collected	50.00%
PORT II OP LP | PORT II OP LP
Related Party Transaction [Line Items]
Payments to acquire investments											$ 5,000,000.0
PORT II OP LP | Tier 1
Related Party Transaction [Line Items]
Base fee, as percent of rent collections per year	8.00%
PORT II OP LP | Tier 2
Related Party Transaction [Line Items]
Base fee, as percent of rent collections per year	7.00%
PORT II OP LP | Tier 3
Related Party Transaction [Line Items]
Base fee, as percent of rent collections per year	6.00%
PORT II OP LP | Maximum | Tier 1
Related Party Transaction [Line Items]
Benchmark of rent collections per year	$ 50,000,000
PORT II OP LP | Maximum | Tier 2
Related Party Transaction [Line Items]
Benchmark of rent collections per year	75,000,000
PORT II OP LP | Minimum | Tier 2
Related Party Transaction [Line Items]
Benchmark of rent collections per year	50,000,000
PORT II OP LP | Minimum | Tier 3
Related Party Transaction [Line Items]
Benchmark of rent collections per year	$ 75,000,000
Investment in PORT II OP LP
Related Party Transaction [Line Items]
Ownership %											96.10%
Pacific Oak Capital Advisors LLC
Related Party Transaction [Line Items]
Term of advisory agreement				1 year
Advisory agreement, renewal period				1 year