ACQUISITIONS AND DISPOSITIONS - Acquisitions (Details)			1 Months Ended		12 Months Ended
	Dec. 29, 2017 USD ($) ft²	Aug. 26, 2015 USD ($) ft²	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) business	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Transaction Costs					$ 11,862,000	$ 340,000	$ 1,382,000
Number of acquisitions | business						0
9460 Wilshire Boulevard in Los Angeles, California
Business Acquisition [Line Items]
Percentage of ownership acquired				100.00%
Escrow Deposit				$ 20,000,000	$ 20,000,000
9460 Wilshire Boulevard in Los Angeles, California - Retail Space
Business Acquisition [Line Items]
Rentable space | ft²				22,884	22,884
9460 Wilshire Boulevard in Los Angeles, California - Office Space
Business Acquisition [Line Items]
Rentable space | ft²				68,866	68,866
Two Kaiser Plaza Parking Lot
Business Acquisition [Line Items]
Percentage of ownership acquired		100.00%
Rentable space | ft²		44,642
Transaction Costs		$ 107,000
Purchase price		$ 11,143,000
Subsequent event | 9460 Wilshire Boulevard in Los Angeles, California
Business Acquisition [Line Items]
Transaction Costs			$ 48,000
Purchase price			$ 132,000,000
1130 Howard Street, San Francisco, CA
Business Acquisition [Line Items]
Percentage of ownership acquired	100.00%
Rentable space | ft²	21,194
Closing costs and assumption of liabilities	$ 1,915,000
Asset acquisitions, purchase price	$ 17,717,000