Real Estate Investments - Summary of Properties Classified as Held for Sale Included in Continuing Operations (Parenthetical) (Details) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2018 USD ($) ft² Parcel	Sep. 30, 2018 USD ($) ft² property Parcel	Dec. 31, 2017 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, carrying value		$ 297,194	$ 297,194	$ 392
Held for Sale Properties Included in Continuing Operations [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, carrying value		$ 297,194	$ 297,194
Held for Sale Properties Included in Continuing Operations [Member] | Metropolitan Washington, D.C. - Office [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property			8
Rentable Square Feet | ft²		1,293,197	1,293,197
Assets held for sale, carrying value	[1]	$ 285,619	$ 285,619
Provision for impairment on assets held for sale		56,900
Impairment Hold Period			10 years
Held for Sale Properties Included in Continuing Operations [Member] | Metropolitan Washington, D.C. - Office [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, carrying value		366,000	$ 366,000
Assets held for sale, fair value		$ 309,100	$ 309,100
Held for Sale Properties Included in Continuing Operations [Member] | Metropolitan Washington, D.C. - Office [Member] | Residual Capitalization Rates [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Measurement input for impairment on assets held for sale		0.0747	0.0747
Held for Sale Properties Included in Continuing Operations [Member] | Metropolitan Washington, D.C. - Office [Member] | Discount Rates [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Measurement input for impairment on assets held for sale		0.0860	0.0860
Held for Sale Properties Included in Continuing Operations [Member] | Pennsylvania Suburbs - Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, carrying value	[2]	$ 4,254	$ 4,254
Number of Parcels | Parcel		1	1
Held for Sale Properties Included in Continuing Operations [Member] | Other - Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, carrying value	[2]	$ 7,321	$ 7,321
Number of Parcels | Parcel		2	2

[1] As of September 30, 2018, the Company determined that the sale of eight office properties, containing 1,293,197 rentable square feet, in the Metropolitan Washington, D.C. segment, was probable and classified these properties as held for sale in accordance with applicable accounting standards for long-lived assets. At such date, the $366.0 million carrying value of the properties exceeded the estimated $309.1 million fair value less the anticipated costs of sale. As a result, the Company recognized an impairment loss totaling approximately $56.9 million during the three-month period ended September 30, 2018. The Company measured this impairment based on a discounted cash flow analysis, using a hold period of ten years and residual capitalization rates and discount rates of 7.47% and 8.60%, respectively. The results were comparable to indicative pricing in the market. As significant inputs to the model are unobservable, the Company determined that the value determined for this property falls within Level 3 fair value reporting.

[2] As of September 30, 2018, the Company determined that the sale of one land parcel in the Pennsylvania Suburbs segment and two parcels of land in the Other segment was probable and classified these properties as held for sale in accordance with applicable accounting standards for long-lived assets. At such date, the fair value less the anticipated costs of sale of the properties exceeded the carrying values. As a result, the Company expects to record gains on sale. The fair value measurement will be based on the pricing in the purchase and sale agreements.