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Organization of The Parent Company and The Operating Partnership - Summary of Core Portfolio of Operating Properties and Excludes Development and Redevelopment Properties Under Construction (Details) | Sep. 30, 2018 ft² property
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Real Estate Properties [Line Items] |  |
Number of Properties | property | 93 |
Rentable Square Feet | ft² | 16,462,764 |
Office Properties [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 73 |
Rentable Square Feet | ft² | 13,673,405 |
Mixed-use Properties [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 4 |
Rentable Square Feet | ft² | 646,741 |
Retail Properties [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 1 |
Rentable Square Feet | ft² | 17,884 |
Core Properties [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 78 |
Rentable Square Feet | ft² | 14,338,030 |
Development Projects [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 2 |
Rentable Square Feet | ft² | 247,818 |
Redevelopment Projects [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 5 |
Rentable Square Feet | ft² | 583,719 |
Held for Sale Properties [Member] |  |
Real Estate Properties [Line Items] |  |
Number of Properties | property | 8 | [1]
Rentable Square Feet | ft² | 1,293,197 | [1]
[1] | As of September 30, 2018, eight office properties were classified as held for sale in the Company’s Metropolitan Washington, D.C. segment. See Note 3, “Real Estate Investments,” for further information.

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