Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating properties		$ 3,586,295		$ 3,693,000		$ 4,603,692
Assets held for sale	[1],[2]	73,591	[3]	794,588	[3]	27,436
Operating Properties		3,659,886		4,487,588		4,631,128
Construction-in-progress		297,462		268,983		201,360
Land held for development		150,970		130,479		90,603
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,320,974		1,157,667		1,338,655
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		1,005,446		1,019,280		1,178,470
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		975,987		1,129,206		1,183,652
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties		146,794		164,518		0
Other [Member]
Segment Reporting Information [Line Items]
Operating properties	[4]	$ 137,094		$ 222,329		$ 902,915

[1] 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.

[2] 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. Additionally, as of December 31, 2016, the Company categorized three office properties located in the Metropolitan Washington, D.C. segment and two properties 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.

[3] 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.

[4] As a result of the Och-Ziff Sale that occurred on February 4, 2016, the Company narrowed its segments to five segments: (1) Pennsylvania Suburbs, (2) Philadelphia Central Business District (“CBD”), (3) Metropolitan Washington, D.C. and (4) Austin, Texas and (5) Other. 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 “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.