Investments in Unconsolidated Joint Ventures (Narrative) (Details)	6 Months Ended
	Jun. 30, 2018 property joint_venture	Dec. 31, 2017 property
Schedule of Equity Method Investments [Line Items]
Number of properties | property	198	205
Properties in Seattle, Hong Kong, Tokyo, Osaka
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	50.00%
Number of joint ventures | joint_venture	4
Prudential Real Estate Investors and Griffin Capital Essential Asset REIT, Inc.
Schedule of Equity Method Investments [Line Items]
Number of joint ventures | joint_venture	2
PREI
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	20.00%
Number of joint ventures | joint_venture	1
Number of properties | property	10
GCEAR
Schedule of Equity Method Investments [Line Items]
Number of properties | property	1
Property in Silicon Valley
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	17.00%