Debt Obligations - Consolidated Debt Obligations Outstanding (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2016	Apr. 02, 2016	Dec. 31, 2015
Consolidated debt obligations
Total Debt Obligations		$ 2,034,621		$ 2,384,717
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		348,100		562,695
Plus: premiums/(discounts), net		(2,979)		(3,198)
Less: deferred financing costs		(813)		(13,744)
Total mortgage indebtedness		344,308		545,753
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		1,703,610		1,853,529
Plus: premiums/(discounts), net		(5,186)		(5,714)
Less: deferred financing costs		(8,111)		(8,851)
Total unsecured indebtedness		1,690,313		1,838,964
Effective interest rate			6.00%
Fairview Eleven Tower [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[1]	$ 20,665		20,838
Effective interest rate	[1]	4.25%
Debt instrument maturity date	[1]	Jan. 01, 2017
Two Logan Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 86,774		86,886
Effective interest rate		3.98%
Debt instrument maturity date		May 01, 2020
One Commerce Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 128,661		130,000
Effective interest rate		3.64%
Debt instrument maturity date		Apr. 05, 2023
Two Commerce Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 112,000		112,000
Effective interest rate	[2]	4.51%
Debt instrument maturity date		Apr. 05, 2023
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[3]	$ 0		177,425
Effective interest rate	[3]	7.00%
Debt instrument maturity date	[3]	Sep. 10, 2030
Cira South Garage [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[3]	$ 0		35,546
Effective interest rate	[3]	7.12%
Debt instrument maturity date	[3]	Sep. 10, 2030
Seven Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 250,000		250,000
Effective interest rate		3.72%
Debt instrument maturity date		Oct. 01, 2022
$250.0M 6.00% Guaranteed Notes due 2016 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross				149,919
Effective interest rate		5.95%
Debt instrument maturity date		Apr. 01, 2016
$300.0M 5.70% Guaranteed Notes due 2017 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 300,000		300,000
Effective interest rate		5.68%
Debt instrument maturity date		Apr. 01, 2016
$325.0M 4.95% Guaranteed Notes due 2018 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 325,000		325,000
Effective interest rate		5.13%
Debt instrument maturity date		May 01, 2017
$250M 3.95% Guaranteed Notes due 2023 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 250,000		250,000
Effective interest rate		4.02%
Debt instrument maturity date		Apr. 15, 2018
250.0M 4.10% Guaranteed Notes due 2024 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 250,000		250,000
Effective interest rate		4.33%
Debt instrument maturity date		Feb. 15, 2023
$250M 4.55% Guaranteed Notes due 2029 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 250,000		250,000
Effective interest rate		4.60%
Debt instrument maturity date		Oct. 01, 2024
Indenture IA (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 27,062		27,062
Effective interest rate		2.75%
Debt instrument maturity date		Mar. 30, 2035
Indenture IB (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 25,774		25,774
Effective interest rate		3.30%
Debt instrument maturity date		Apr. 30, 2035
Indenture II (Preferred Trust II) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 25,774		$ 25,774
Effective interest rate		3.09%
Debt instrument maturity date		Jul. 30, 2035

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

[2] This loan was assumed upon acquisition of Two Commerce Square on December 19, 2013. The interest rate reflects the market rate at the time of acquisition. A default under this loan will also constitute a default under the loan outstanding on One Commerce Square, and a default under the One Commerce Square loan will also constitute a default under this loan. This loan is also secured by a lien on One Commerce Square and the loan on Two Commerce Square.

[3] (a) On January 14, 2016, the Company funded $265.8 million to prepay two mortgage loans, consisting of $176.9 million of principal repayment, $44.5 million in prepayment charges and a nominal amount of accrued interest, in repayment of the mortgage indebtedness on the office property located at 2970 Market Street in Philadelphia, Pennsylvania commonly known as 30th Street Main Post Office (“Cira Square”), ahead of its scheduled maturity date of September 10, 2030. Also on January 14, 2016, the Company funded $44.4 million, consisting of $35.5 million of principal repayment, $8.9 million in prepayment charges and a nominal amount of accrued interest, in repayment of the mortgage indebtedness of a 1,662 parking space facility located in Philadelphia, Pennsylvania commonly known as (“Cira South Garage”), ahead of its scheduled maturity date of September 10, 2030.  These repayments were financed with $195.0 million of funds available under the unsecured revolving credit facility with the remaining balance funded through available cash balances. The Company recognized a $66.6 million loss on extinguishment of debt, consisting of the prepayment charges along with $10.8 million and $2.4 million related to non-cash charges for deferred financing costs for Cira Square and Cira South Garage, respectively. (a) On April 1, 2016, the entire principal balance of the 2016 6.00% Guaranteed Unsecured Notes was repaid upon maturity. Available cash balances were used to fund the repayment of the unsecured notes.