Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details) $ in Thousands				3 Months Ended		6 Months Ended
		Apr. 07, 2016 USD ($) ft²	Jan. 14, 2016 USD ($) ft²	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Apr. 02, 2016	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Mortgage notes payable, net				$ 344,308		$ 344,308			$ 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				348,100		348,100			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%
Fairview Eleven Tower [Member]
Debt Instrument [Line Items]
Percentage of interest maintained in mortgage debt				50.00%		50.00%
Fairview Eleven Tower [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	[1]			$ 20,665		$ 20,665			$ 20,838
Debt instrument maturity date	[1]					Jan. 01, 2017
Effective interest rate	[1]			4.25%		4.25%

[1] Represents the full debt amount of a property in a consolidated real estate venture in which the Company holds a 50% interest.