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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
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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

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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
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