Segment Information Summary of Real Estate Investments (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating properties		$ 4,629,223	$ 4,603,692
Less: Assets held for sale	[1]	74,706	27,436
Construction-in-progress		242,246	201,360
Land inventory		135,917	90,603
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating properties		4,629,223	4,603,692
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,394,267	1,338,655
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Operating properties		1,126,611	1,178,470
Operating Segments [Member] | Metropolitan Dc [Member]
Segment Reporting Information [Line Items]
Operating properties		1,216,161	1,183,652
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Operating properties	[2]	239,235	392,581
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Operating properties		311,642	317,076
Operating Segments [Member] | California
Segment Reporting Information [Line Items]
Operating properties		111,587	193,258
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties	[3]	229,720	0
Operating Segments [Member] | Assets Held-for-sale [Member]
Segment Reporting Information [Line Items]
Less: Assets held for sale	[2],[4]	74,706	27,436
Operating Segments [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating properties		4,703,929	4,631,128
Operating Segments [Member] | Corporate Segment [Member]
Segment Reporting Information [Line Items]
Construction-in-progress		242,246	201,360
Land inventory		$ 135,917	$ 90,603

[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.

[2] As of September 30, 2015, the Company categorized six properties in the New Jersey/Delaware segment as held for sale in accordance with applicable accounting standards for long lived assets. The sale is not classified as a significant disposition under the accounting guidance for discontinued operations.

[3] 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 September 30, 2015. See Note 3, "Real Estate Investments," for further information regarding this transaction.

[4] On December 31, 2014, the Company was actively marketing for sale its Atrium I and Libertyview properties, comprised of two office properties located in the New Jersey/Delaware segment. As of December 31, 2014 the properties were classified as held for sale on the consolidated balance sheet. The properties were sold on January 8, 2015. See Note 3, "Real Estate Investments," for further information. The sale is not classified as a significant disposition under the accounting guidance for discontinued operations.