Segment Information - Real Estate Investments (Details) - USD ($) $ in Thousands		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating properties		$ 3,693,000		$ 4,603,692		$ 4,669,289
Assets held for sale		794,588	[1]	27,436	[1]	0
Construction-in-progress		268,983		201,360
Land inventory		130,479		90,603
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating properties		3,693,000		4,603,692		4,669,289
Operating Properties		4,487,588		4,631,128		4,669,289
Operating Segments [Member] | Held for Sale Properties Included in Continuing Operations [Member]
Segment Reporting Information [Line Items]
Assets held for sale	[2],[3]	794,588		27,436		0
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties	[2]	1,157,667		1,338,655		1,300,666
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Operating properties	[3]	1,019,280		1,178,470		1,199,105
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties	[3]	1,129,206		1,183,652		1,214,965
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Operating properties	[3]	154,326		392,581		414,716
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Operating properties	[3]	0		317,076		310,397
Operating Segments [Member] | California [Member]
Segment Reporting Information [Line Items]
Operating properties		68,003		193,258		192,584
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties	[4]	164,518		0		36,856
Corporate [Member]
Segment Reporting Information [Line Items]
Construction-in-progress		268,983		201,360		74,174
Land inventory		$ 130,479		$ 90,603		$ 93,351

[1] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation or other assets on the balance sheets of the properties held for sale. See Held for Sale section below.

[2] As of December 31, 2015, 2970 Market Street was classified as held for sale on the consolidated balance sheets. The property was sold on February 5, 2016. See Note 21, "Subsequent Events," for further information. The sale is not classified as a significant disposition under the accounting guidance for discontinued operations.

[3] As of December 31, 2015, the 58 properties associated with the transaction with Och Ziff Real Estate were classified as held for sale on the consolidated balance sheets. On February 4, 2016, the Company completed a series of transactions resulting in the disposition of the properties. See Note 21, “Subsequent Events,” for further information regarding the disposition.

[4] On June 22, 2015 the Company acquired the remaining 50.0% of the common interest in Broadmoor Austin Associates. As such, the Company has seven wholly owned properties in its Austin, Texas business segment at December 31, 2015. See Note 3, "Real Estate Investments," for further information regarding this transaction.