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
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.