Real Estate Investments - Gross Carrying Value of Properties (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment, Gross [Abstract]
Land		$ 497,732	$ 513,268
Tenant improvements	[1]	463,101	459,952
Real Estate Investments, and Building and improvements		3,721,405	3,693,000
Assets held for sale - real estate investments	[2],[3],[4]	17,622	794,588
Total		3,739,027	4,487,588
Building and Improvements [Member]
Property, Plant and Equipment, Gross [Abstract]
Real Estate Investments, and Building and improvements	[1]	$ 2,760,572	$ 2,719,780

[1] The net increase primarily relates to the office component of the FMC Tower at Cira Centre South in Philadelphia, Pennsylvania being placed into service during the three-month period ended June 30, 2016, offset partially by the classification of 1120 Executive Boulevard in Mount Laurel, New Jersey as held for sale as of June 30, 2016.

[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. 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.

[3] 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.

[4] 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.