Real Estate Investments - Dispositions (Textual) (Details)		3 Months Ended					6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) property	Sep. 30, 2018 USD ($) ft²	Dec. 31, 2017 USD ($) a Parcel	Mar. 31, 2017 USD ($) property	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) property	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) ft² property	Dec. 31, 2018 USD ($) ft² a property Parcel	Dec. 31, 2017 USD ($) a Parcel	Dec. 31, 2016 USD ($)	Dec. 31, 2016	Dec. 31, 2016 property	Dec. 31, 2016 a	Dec. 31, 2016 Parcel	Dec. 31, 2016 Office
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property									97
Aggregate carrying values of properties			$ 3,832,348,000						$ 3,953,319,000	$ 3,832,348,000
Number of Properties Sold | property									10
Rentable Square Feet | ft²									1,414,457
Assets held for sale, net			$ 392,000						$ 11,599,000	392,000
Net gain on disposition of real estate									2,932,000	31,657,000	$ 116,983,000
Impairment charges					$ 1,800,000				$ 71,707,000	$ 3,057,000	40,517,000
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Acreage of land | a			76.4						19.7	76.4				122.9
Number of Parcels | Parcel			5						2	5					3
Metropolitan Washington, D.C. - Office [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale		$ 56,900,000
2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain on disposition of real estate				$ 500,000
Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, net									$ 11,599,000		41,718,000
Impairment charges									$ 0
Acreage of land | a			13.1							13.1
Assets Held-for-sale [Member] | Land Held For Development [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, net			$ 400,000							$ 400,000
Assets Held-for-sale [Member] | Metropolitan Washington, D.C. - Office [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment Hold Period								10 years
Number of Properties Sold | property								8
Rentable Square Feet | ft²		1,293,197						1,293,197
Assets held for sale, net											9,785,000
Provision for impairment on assets held for sale								$ 56,900,000
Assets Held-for-sale [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, net		$ 366,000,000.0						366,000,000.0
Assets held for sale, fair value		$ 309,100,000						$ 309,100,000
Residual Capitalization Rates [Member] | Assets Held-for-sale [Member] | Metropolitan Washington, D.C. - Office [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
Discount Rates [Member] | Assets Held-for-sale [Member] | Metropolitan Washington, D.C. - Office [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
Other [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties	4			4					1				3			2
Impairment of properties held for use	$ 1,000,000.0								$ 14,800,000		7,300,000
Carrying value of real estate prior to impairment			10,200,000						$ 52,800,000	10,200,000	$ 25,800,000
Impairment Hold Period	10 years								10 years		10 years
Impairment charges			0
Aggregate carrying value											$ 18,500,000
Other [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of properties held for use											5,600,000
Carrying value of real estate prior to impairment											18,200,000
Impairment charges			0				$ 300,000
Acreage of land | a														108
Aggregate carrying value											$ 12,600,000
Number of Parcels | Parcel															5
Other [Member] | Impairment on Held for Use Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Aggregate carrying values of properties			$ 9,200,000						$ 38,000,000	$ 9,200,000
Other [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	0.0900			0.0900					0.075			0.0875
Other [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	0.0925			0.0925					0.095			0.0900
Metropolitan DC [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties						2										3
Impairment of properties held for use					3,900,000	$ 7,400,000
Carrying value of real estate prior to impairment					$ 37,400,000	$ 51,900,000
Impairment Hold Period					10 years	10 years
Aggregate carrying value					$ 33,500,000	$ 44,500,000
Metropolitan DC [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					0.0775	0.070
Metropolitan DC [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					0.0825	0.070