ACQUISITIONS AND DISPOSITIONS - Acquisitions (Details) $ in Thousands			12 Months Ended
	Jan. 18, 2018 USD ($) ft²	Dec. 29, 2017 USD ($) ft²	Dec. 31, 2016 business	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | business			0
9460 Wilshire Boulevard in Los Angeles, California
Business Acquisition [Line Items]
Percentage of ownership acquired	100.00%
Rentable space (in sq ft) | ft²	91,750
Purchase price	$ 132,000
Escrow deposit				$ 20,000
Closing costs and assumption of liabilities					$ 48
9460 Wilshire Boulevard in Los Angeles, California - Office Space
Business Acquisition [Line Items]
Rentable space (in sq ft) | ft²				68,866
9460 Wilshire Boulevard in Los Angeles, California - Retail Space
Business Acquisition [Line Items]
Rentable space (in sq ft) | ft²				22,884
1130 Howard Street, San Francisco, CA
Business Acquisition [Line Items]
Percentage of ownership acquired		100.00%
Rentable space (in sq ft) | ft²		21,194
Purchase price		$ 17,717
Closing costs and assumption of liabilities		$ 1,915