Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating properties		$ 3,721,405	$ 3,693,000
Assets held for sale	[1],[2],[3]	17,622	794,588
Total		3,739,027	4,487,588
Construction-in-progress		254,188	268,983
Land held for development		131,015	130,479
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties	[4]	1,274,818	1,157,667
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		1,022,239	1,019,280
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties	[5]	1,068,170	1,129,206
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties	[6]	147,157	164,518
Other [Member]
Segment Reporting Information [Line Items]
Operating properties	[7],[8]	$ 209,021	$ 222,329

[1] As of December 31, 2015, the 58 properties associated with the series of related transactions 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 3, “Real Estate Investments,” for further information regarding the disposition. The sale is not classified as a significant disposition under the accounting guidance for discontinued operations. Additionally, as of June 30, 2016, the Company categorized one office property located at 1120 Executive Boulevard in Mt. Laurel, New Jersey and one property located in Oakland, California known as Oakland Lot B 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.

[2] As of December 31, 2015, the office property located at 2970 Market Street in Philadelphia, Pennsylvania commonly known as 30th Street Main Post Office (“Cira Square”) was classified as held for sale on the consolidated balance sheets. The Company disposed of its interests in the property on February 5, 2016. 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.

[3] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation or intangible assets on the balance sheets of the properties held for sale.

[4] The increase primarily relates to the office component of the FMC Tower at Cira Centre South being placed into service during the three-month period ended June 30, 2016.

[5] The decrease primarily relates to the sale of Herndon Metro Plaza I & II. 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.

[6] The decrease primarily relates to a building from the Broadmoor Austin portfolio being placed into redevelopment during the three-month period ended June 30, 2016.

[7] As a result of the Och-Ziff Sale that occurred on February 4, 2016, the Company narrowed its segments to five segments located in: (1) Pennsylvania Suburbs, (2) Philadelphia Central Business District (“CBD”), (3) Metropolitan Washington, D.C. and (4) Austin, Texas. The Och-Ziff Sale disposed of the entire Richmond, Virginia segment. Subsequent to the Och-Ziff Sale, the segments previously defined as New Jersey/Delaware and California are now being managed as a consolidated segment entitled (5) “Other,” as these geographies no longer provide a significant revenue contribution. Accordingly, the chief operating decision maker revised the management structure, reallocated resources, and is assessing business operations of the five segments as of January 1, 2016.

[8] The decrease primarily relates to the office property at 1120 Executive Boulevard in Mount Laurel, New Jersey and Oakland Lot B in Oakland, California being classified as held for sale at June 30, 2016. See Note 3, "Real Estate Investments," for further information. These sales are not classified as significant dispositions under the accounting guidance for discontinued operations.