Acquisitions - Development Project Acquisitions and Acquisition Costs (Details) $ in Thousands				12 Months Ended
	Dec. 12, 2019 USD ($)	Aug. 19, 2019 USD ($)	Jun. 01, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Purchase price				$ 173,000	$ 308,200
Asset acquisition, accrued liabilities				20,890	8,921
Restricted cash				16,300	119,430	$ 9,149	$ 56,711
Payments to acquire land parcel				99,500
Acquisition costs, capitalized				1,600	$ 3,800	$ 4,600
1335 Broadway & 901 Park Boulevard, San Diego, CA [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Purchase price		$ 40,000
Asset acquisition, accrued liabilities		$ 4,000
Seattle CBD Project [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Purchase price	$ 133,000
Asset acquisition, accrued liabilities				6,300
Restricted cash				$ 10,000
Kilroy Oyster Point [Member]
Schedule of Asset Acquisitions, by Acquisition [Line Items]
Number of buildings | property					3
Purchase price			$ 308,200
Asset acquisition, accrued liabilities			$ 40,600