Segment Information - Net Operating Income of Company's Reportable Segments (Additional Information) (Details)	Sep. 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	95
Net rentable area (in square feet) | ft²	16,711,387
Herndon Innovation Center Metro Portfolio Venture, LLC
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage		15.00%
Herndon Innovation Center Metro Portfolio Venture, LLC | Metropolitan Washington, D.C. - Office
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage			15.00%
Net rentable area (in square feet) | ft²			1,293,197
Number of properties contributed to a joint venture | property			8
Herndon Innovation Center Metro Portfolio Venture, LLC | Metropolitan Washington, D.C. - Office | Rockpoint
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage			85.00%
DRA Austin Venture
Segment Reporting Information [Line Items]
Equity method investment, ownership percentage				50.00%
Number of properties | property				12
Net rentable area (in square feet) | ft²				1,570,123