Acquisitions - Operating Property Acquisitions (Details) $ in Millions				12 Months Ended
	Oct. 15, 2019 USD ($) ft² building	Dec. 21, 2018 USD ($) ft² building	Jan. 31, 2018 USD ($) ft² building	Dec. 31, 2019 USD ($) ft² building	Dec. 31, 2018 USD ($) ft² building
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Rentable Square Feet (unaudited) | ft²				13,475,795
Purchase price				$ 173.0	$ 308.2
Revenue contributed from acquired properties				3.7	8.0
Net income (loss) contributed from acquired properties				$ (0.1)	$ 1.7
3101-3243 La Cienega Boulevard, Culver City, CA [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Number of buildings | building	19			19
Rentable Square Feet (unaudited) | ft²	151,908
Occupancy percent				100.00%
Purchase price	$ 186.0
345, 347 & 349 Oyster Point Boulevard, South San Francisco, CA [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Number of buildings | building			3
Rentable Square Feet (unaudited) | ft²			145,530
Occupancy percent				100.00%
Purchase price			$ 111.0
345 Brannan Street, San Francisco, CA [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Number of buildings | building		1
Rentable Square Feet (unaudited) | ft²		110,050
Occupancy percent				99.70%
Purchase price		$ 146.0
345, 347 & 349 Oyster Point Boulevard, South San Francisco, CA and 345 Brannan Street, San Francisco, CA [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Number of buildings | building					4
Rentable Square Feet (unaudited) | ft²					255,580
Purchase price					$ 257.0