Real Estate Investments - Dispositions (Textual) (Details)		3 Months Ended					6 Months Ended	9 Months Ended
		Sep. 30, 2017 USD ($) a property	Jun. 30, 2017 USD ($) a Parcel	Mar. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($) a property	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) a Parcel	Sep. 30, 2017 USD ($) a property	Sep. 30, 2016 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate		$ 0				$ (104,000)		$ 8,411,000	$ 114,625,000
Number of Properties | property		97						97
Assets held for sale, net		$ 31,125,000			$ 41,718,000			$ 31,125,000
Impairment charges		0				$ 0		3,057,000	$ 13,069,000
Aggregate carrying values of properties		$ 3,769,067,000			3,586,295,000			3,769,067,000
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate								$ 953,000
Acreage of land | a		76.4						76.4
Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property		6						6
Assets held for sale, net		$ 31,125,000						$ 31,125,000
Assets Held-for-sale [Member] | Operating Real Estate Investments, Net [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, net					40,700,000
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					$ 1,000,000
Other Segments
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | segment				4
Impairment charges		0
Carrying value of real estate prior to impairment		10,200,000						10,200,000
Impairment of properties held for use				$ 1,000,000
Impairment Hold Period				10 years
Residual Capitalization Rates				9.00%
Discount Rates				9.25%
Other Segments | 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						9,200,000
Other Segments | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment charges		$ 0
Other Segments | Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale								0
Acreage of land | a					5
2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate				$ 500,000
Bishops Gate [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate	[1]							$ 71,000
Provision for impairment on assets held for sale			$ 300,000		$ 3,000,000
Acreage of land | a		49.5						49.5
Bishops Gate [Member] | Other Segments | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale							$ 300,000
Acreage of land | a			50				50
Number of Parcels | Parcel			1				1
Carrying value of real estate prior to impairment			$ 5,900,000				$ 5,900,000
Aggregate carrying value			$ 5,600,000				$ 5,600,000
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property								12
Number of Properties | property		82						82
Office Properties [Member] | Other Segments | Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property					2
Office Properties [Member] | Metropolitan DC [Member] | Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property					3
Retail Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property								1
Mixed-use Property [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property								1

[1] During the fourth quarter of 2016, the Company recognized an impairment of $3.0 million. During the second quarter of 2017, the Company recognized an additional impairment of $0.3 million, representing closing costs. See “Land Impairment” section below.