Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details) $ in Thousands				3 Months Ended		9 Months Ended
		Apr. 07, 2016 USD ($) ft²	Jan. 14, 2016 USD ($) ft²	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Apr. 02, 2016	Dec. 31, 2015 USD ($)	Dec. 20, 2011
Debt Instrument [Line Items]
Mortgage notes payable, net				$ 322,623		$ 322,623			$ 545,753
Loss on early extinguishment of debt - deferred financing costs						13,157	$ 0
Loss on early extinguishment of debt			$ (66,600)	0	$ 0	(66,590)	$ 0
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Repayments of debt			195,000
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				326,242		326,242			562,695
Secured Debt [Member] | Cira Square and Cira South Garage [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount			265,800
Secured Debt [Member] | Cira Square [Member]
Debt Instrument [Line Items]
Repayments of debt			176,900
Repayment of prepayment charges			$ 44,500
Debt instrument maturity date			Sep. 10, 2030
Loss on early extinguishment of debt - deferred financing costs			$ 10,800
Secured Debt [Member] | Cira South Garage [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount			44,400
Repayments of debt			35,500
Repayment of prepayment charges			$ 8,900
Debt instrument maturity date			Sep. 10, 2030
Number of Parking Spaces | ft²			1,662
Loss on early extinguishment of debt - deferred financing costs			$ 2,400
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 1,703,610		$ 1,703,610			1,853,529
Effective interest rate								6.00%
First Mortgage [Member | Mortgage Loans over $1,000,000 [Member] | Office Building at Two Logan Square [Member]
Debt Instrument [Line Items]
Mortgage notes payable, net		$ 86,900
Net Rentable Square Feet | ft²		708,844
Former First Mortgage [Member] | Mortgage Loans over $1,000,000 [Member] | Office Building at Two Logan Square [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 86,600
Loan interest rate		7.57%
Brandywine - AI Venture LLC [Member]
Debt Instrument [Line Items]
Equity method investment percentage										50.00%
3141 Fairview Eleven Tower [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	[1]								$ 20,838
Debt instrument maturity date	[1]					Jan. 01, 2017
Effective interest rate	[1]			4.25%		4.25%
3141 Fairview Eleven Tower [Member] | Brandywine - AI Venture LLC [Member]
Debt Instrument [Line Items]
Equity method investment percentage				50.00%		50.00%

[1] On August 31, 2016, the Company deconsolidated 3141 Fairview Park Drive and began accounting for it under the equity method of accounting as part of Brandywine - AI Venture LLC, an unconsolidated real estate venture in which the Company holds a 50% interest. At December 31, 2015, this balance represented the full debt amount of the property, as it was consolidated at that time. See Note 4, “Investment in Unconsolidated Real Estate Ventures,” for further details.