Real Estate Investments - Dispositions (Textual) (Details)			3 Months Ended				6 Months Ended	12 Months Ended
		Mar. 31, 2017 USD ($) segment	Dec. 31, 2017 USD ($) a property Parcel	Mar. 31, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) property	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) a property Parcel	Dec. 31, 2016 USD ($) a property Parcel	Dec. 31, 2015 USD ($) a property Parcel
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain on disposition of real estate								$ 31,657,000	$ 116,983,000	$ 20,496,000
Impairment charges					$ 1,800,000			$ 3,057,000	40,517,000	82,208,000
Number of Properties | property			93					93
Aggregate carrying values of properties			$ 3,832,348,000					$ 3,832,348,000	3,586,295,000
Assets held for sale, net			$ 392,000					$ 392,000	41,718,000
Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Acreage of land | a			13.1					13.1
Assets held for sale, net									$ 41,718,000	$ 584,365,000
Land Held For Development [Member] | 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			$ 400,000					$ 400,000
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Acreage of land | a			76.4					76.4	122.9	14.7
Number of Parcels | Parcel			5					5	3	4
Other Segments
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment charges			$ 0
Number of Properties		4		4					3
Impairment of properties held for use		$ 1,000,000							$ 7,300,000
Carrying value of real estate prior to impairment			10,200,000					$ 10,200,000	$ 25,800,000
Impairment Hold Period		10 years							10 years
Residual Capitalization Rates		9.00%							8.75%
Discount Rates		9.25%							9.00%
Aggregate carrying value									$ 18,500,000
Other Segments | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment charges			0				$ 300,000
Impairment of properties held for use									5,600,000
Carrying value of real estate prior to impairment									$ 18,200,000
Acreage of land | a									108
Aggregate carrying value									$ 12,600,000
Number of Parcels | Parcel									5
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
Metropolitan DC [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						2				3
Impairment of properties held for use					3,900,000	$ 7,400,000				$ 27,500,000
Carrying value of real estate prior to impairment					$ 37,400,000	$ 51,900,000
Impairment Hold Period					10 years	10 years				10 years
Residual Capitalization Rates					7.75%	7.00%				8.00%
Discount Rates					8.25%	7.00%				8.00%
Aggregate carrying value					$ 33,500,000	$ 44,500,000				$ 40,400,000
Estimated Fair Value Of Impaired Property										12,900,000
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
2970 Market Street (Cira Square) [Member] | 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	[1]									$ 234,025,000

[1] On December 23, 2015 the Company entered into a purchase and sale agreement to dispose of its equity interests in the office property located at 2970 Market Street in Philadelphia commonly known as 30th Street Main Post Office (“Cira Square”), which includes 862,692 square feet of rentable space and is fully leased to a single tenant. As of December 31, 2015, the Company determined the sale was probable and classified the property as held for sale in accordance with applicable accounting standards for long lived assets. As the fair value less anticipated costs to sell exceeded the carrying value of the property no impairment loss was recorded. The fair value measurement was based on the pricing in the purchase and sale agreement. As the sales price is unobservable, the Company determined that the significant inputs used to value this real estate investment are Level 3 in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, “Fair Value Measurements and Disclosures.” On February 5, 2016 the Company completed the disposition of its equity interests in Cira Square.