Segment Information - Net Operating Income of Company's Reportable Segments (Parenthetical) (Details)	Jun. 30, 2019 ft² property	Mar. 29, 2019	Dec. 20, 2018 ft² property	Dec. 11, 2018 ft² property
Segment Reporting Information [Line Items]
Number of Properties | property	96
Area of Real Estate Property | ft²	16,922,019
Herndon Innovation Center Metro Portfolio Venture, LLC [Member]
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage		15.00%
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member]
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage			15.00%
Area of Real Estate Property | ft²			1,293,197
Number of properties contributed to a joint venture | property			8
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] | Rockpoint [Member]
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage			85.00%
DRA Austin Venture [Member]
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage				50.00%
Number of Properties | property				12
Area of Real Estate Property | ft²				1,570,123