Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands										3 Months Ended										5 Months Ended	12 Months Ended
		Oct. 13, 2017 USD ($) ft²	Jul. 28, 2017 USD ($) ft²	Jul. 02, 2016 USD ($) a	Jul. 07, 2015 USD ($) a	Jun. 22, 2015 USD ($) ft² a property	May 12, 2015 USD ($) ft²	Apr. 06, 2015 USD ($) a	Apr. 02, 2015 USD ($) ft²	Dec. 31, 2017 USD ($) a		Dec. 31, 2017 USD ($) a	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) ft²	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) a	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($) ft²	Jan. 30, 2017 a	Jul. 01, 2016 a
Business Acquisition [Line Items]
Revenue										$ 133,344	[1]		$ 128,438	$ 127,791	$ 130,920	$ 132,086	$ 129,694	$ 127,181	$ 136,502		$ 520,493	$ 525,463	$ 602,631
Net losses										73,137			$ 18,803	$ 4,092	$ 19,278	(14,058)	$ 6,022	$ (3,105)	$ 44,091		120,850	40,191	(30,401)
Amount funded by corporate funds																					72,523	20,406	150,472
Net gain from remeasurement of investments in real estate ventures																					0	0	758
Land held for development	[2]									98,242		$ 98,242				150,970				$ 98,242	98,242	150,970	$ 130,479
2100 Market Street [Member]
Business Acquisition [Line Items]
Acreage of land | a					0.8
Payments to Acquire Land					$ 18,800
Amount funded by corporate funds					16,800
Amount Deferred					$ 2,000
25 M Street [Member]
Business Acquisition [Line Items]
Capitalized transaction costs								$ 300
Area of Real Estate Property | ft²							271,000
Acreage of land | a								0.8
Payments to Acquire Land								$ 20,300
Land held for development										$ 23,100		23,100				$ 22,400				23,100	$ 23,100	$ 22,400
25 M Street [Member] | Brandywine 25 M Street [Member]
Business Acquisition [Line Items]
Percent of Acquired Entity							95.00%
Garza Land Sale [Member]
Business Acquisition [Line Items]
Area of land under agreement to sell | a																								1.7
Office Properties [Member]
Business Acquisition [Line Items]
Rentable Square Feet | ft²																5,239,632						5,239,632	2,649,926
One Drexel Plaza [Member]
Business Acquisition [Line Items]
Lease agreement term		99 years
Rentable Square Feet | ft²		282,709
Gross purchase value of office property		$ 35,000
Capitalized transaction costs		2,800
Revenue												1,200
Net losses												$ (400)
Building, land and improvements		$ 30,583
3000 Market Street [Member]
Business Acquisition [Line Items]
Gross purchase value of office property			$ 32,700
Capitalized transaction costs			700
Revenue																				800
Net losses																				$ (500)
Contractual sale price of real estate acquired			32,000
Building, land and improvements			$ 32,004
3000 Market Street [Member] | Office Properties [Member]
Business Acquisition [Line Items]
Area of Real Estate Property | ft²			58,587
Garza Land Acquisition [Member]
Business Acquisition [Line Items]
Acreage of land | a				34.6																					34.6
Gross purchase value of assets				$ 20,600
Acquisition costs				1,900
Amount funded by corporate funds				$ 20,400
Area of land under agreement to sell | a										9.5		9.5								9.5	9.5
Broadmoor Austin Associates [Member]
Business Acquisition [Line Items]
Rentable Square Feet | ft²						1,112,236
Revenue																					$ 25,400	$ 26,400	$ 13,200
Net losses																					$ 6,900	$ (5,200)	$ (7,200)
Acreage of land | a						66.0
Payments to Acquire Land						$ 143,800
Amount funded by corporate funds						$ 81,000
Percentage of ownership interests						50.00%
Number of Property units | property						7
Purchase Price of Assets						$ 211,400
Carrying amount of Investment						$ 66,300
Lease percentage of acquired property						100.00%
Funds Held in Escrow						$ 62,800
Mortgage Debt Paid at Closing						51,200
Acquisition Costs						$ 200
Ownership percentage						50.00%
Net gain from remeasurement of investments in real estate ventures						$ 800
Building, land and improvements						$ 163,271
25 M Street [Member] | 25 M Street [Member]
Business Acquisition [Line Items]
Payments to Acquire Land							$ 1,000
Asset Acquisition Ownership Percent							5.00%
618 Market Street [Member]
Business Acquisition [Line Items]
Area of Real Estate Property | ft²									14,404
Payments to Acquire Land									$ 19,400
Number of Parking Spaces | ft²									330
Fair Value of Contingent Consideration									$ 1,600
Consideration, Cash									17,800
Building, land and improvements									19,200
Intangible Assets									$ 200

[1] The increase in fourth quarter revenues primarily relates to the operating performance of 1900 Market Street in Philadelphia, Pennsylvania and the acquisition of One Drexel Plaza in Philadelphia, Pennsylvania.

[2] As of December 31, 2017, the Company categorized 13.1 acres of land held for development located in the Other segment as held for sale in accordance with applicable accounting standards for long lived assets. See Note 3, “Real Estate Investments,” for further information.