Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details)					12 Months Ended
		Apr. 07, 2016 USD ($) ft²	Jan. 14, 2016 USD ($) ft²	Dec. 29, 2015 USD ($)	Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Apr. 02, 2016	Dec. 20, 2011
Debt Instrument [Line Items]
Mortgage notes payable, net					$ 321,549,000		$ 545,753,000
Extinguishment of Debt, Amount					0		0	$ 376,174,000
Loss on early extinguishment of debt - deferred financing costs								152,000
Loss on early extinguishment of debt			$ (66,600,000)		(66,590,000)		0	$ (7,594,000)
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Repayments of debt			195,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross					325,038,000		562,695,000
Extinguishment of Debt, Amount			44,400,000
Repayments of debt			35,500,000
Repayment of prepayment charges			8,900,000
Secured Debt [Member] | Cira Square and Cira South Garage [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount			$ 265,800,000
Secured Debt [Member] | Cira Square [Member]
Debt Instrument [Line Items]
Debt instrument maturity date			Sep. 10, 2030
Repayments of debt			$ 176,900,000
Repayment of prepayment charges			44,500,000
Loss on early extinguishment of debt - deferred financing costs			$ 10,800,000
Secured Debt [Member] | Cira South Garage [Member]
Debt Instrument [Line Items]
Debt instrument maturity date			Sep. 10, 2030
Extinguishment of Debt, Amount			$ 44,400,000
Repayments of debt			35,500,000
Repayment of prepayment charges			$ 8,900,000
Number of Parking Spaces | ft²			1,662
Loss on early extinguishment of debt - deferred financing costs			$ 2,400,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 1,703,610,000		1,853,529,000
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		$ 86,900,000
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		$ 86,600,000
Loan interest rate		7.57%
Brandywine - AI Venture LLC [Member]
Debt Instrument [Line Items]
Equity method investment percentage					50.00%	[1],[2]				50.00%
Effective interest rate	[2]				3.96%
3141 Fairview Eleven Tower [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	[3]				$ 0		20,838,000
Debt instrument maturity date	[3]				Jan. 01, 2017
Effective interest rate	[3]				4.25%
3141 Fairview Eleven Tower [Member] | Brandywine - AI Venture LLC [Member]
Debt Instrument [Line Items]
Equity method investment percentage					50.00%
One Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 127,026,000		$ 130,000,000
Debt instrument increase in principal				$ 130,000,000
Debt instrument maturity date					Apr. 05, 2023
Effective interest rate					3.64%	[4]	3.64%

[1] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.

[2] See “Brandywine AI Venture: 3141 Fairview Park Drive” section below for information discussing activity that occurred during 2016 relating to this venture.

[3] 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.

[4] This loan was assumed upon acquisition of the related properties on December 19, 2013. On December 29, 2015, the Company refinanced the debt increasing the principal balance to $130.0 million and extended the term from the scheduled maturity from January 6, 2016 to April 5, 2023. The effective interest rate as of December 31, 2015 was 3.64%. A default under this loan will also constitute a default under the loan outstanding on Two Commerce Square. This loan is also secured by a lien on Two Commerce Square.