Debt Obligations - Consolidated Debt Obligations Outstanding (Details) - USD ($) $ in Thousands			1 Months Ended	9 Months Ended	12 Months Ended
		Oct. 08, 2015	Sep. 30, 2017	Oct. 07, 2015	Dec. 31, 2017		Nov. 09, 2017	Dec. 31, 2016
Consolidated debt obligations
Plus: premiums/(discounts), net					$ (6,748)
Less: deferred financing costs					(11,141)
Total Debt Obligations					1,930,828			$ 2,013,112
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross					320,107			325,038
Plus: premiums/(discounts), net					(2,325)			(2,761)
Less: deferred financing costs					(566)			(728)
Total mortgage indebtedness					317,216			321,549
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross					1,628,610			1,703,610
Plus: premiums/(discounts), net					(4,423)			(4,678)
Less: deferred financing costs					(10,575)			(7,369)
Total unsecured indebtedness					1,613,612			1,691,563
Two Logan Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross					$ 84,440			86,012
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					$ 123,667			127,026
Effective interest rate	[1]				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
$300.0M 5.70% Guaranteed Notes due 2017 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[3]				$ 0			300,000
Effective interest rate	[3]				5.68%
Debt instrument maturity date	[3]				May 01, 2017
$325.0M 4.95% Guaranteed Notes due 2018 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[4]				$ 0			325,000
Effective interest rate	[4]				5.13%
Debt instrument maturity date	[4]				Apr. 15, 2018
$250M 3.95% Guaranteed Notes due 2023 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[4]				$ 350,000			250,000
Effective interest rate	[4]				3.87%
Debt instrument maturity date	[4]				Feb. 15, 2023
Seven Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross		$ 250,000		$ 200,000	$ 250,000			250,000
Effective interest rate		3.72%			3.72%
Debt instrument maturity date		Oct. 08, 2022		Feb. 01, 2019	Oct. 01, 2022
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					Oct. 01, 2024
$450.0M 3.95% Guaranteed Notes due 2027 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross					$ 450,000	[4]	$ 450,000	0	[4]
Effective interest rate	[4]				4.03%
Debt instrument maturity date	[4]				Nov. 30, 2027
$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, 2029
Indenture IA (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	[5]				$ 27,062			27,062
Debt instrument, description of variable rate basis			LIBOR + 1.25%		LIBOR + 1.25%	[5]
Spread on variable rate			1.25%		1.25%	[5]
Debt instrument maturity date	[5]				Mar. 30, 2035
Indenture IB (Preferred Trust I) - Swapped to fixed [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) - Swapped to fixed [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] 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 scheduled maturity date from January 6, 2016 to April 5, 2023.  A default under this loan will also constitute a default under the loan secured by Two Commerce Square. This loan is also secured by a lien on Two Commerce Square.

[2] This loan was assumed upon acquisition of the related property 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 secured by One Commerce Square. This loan is also secured by a lien on One Commerce Square.

[3] On May 1, 2017, the entire principal balance of the unsecured 5.70% Guaranteed Notes was repaid upon maturity. Available cash balances and the Credit Facility (as defined below) were used to fund the repayment of the unsecured notes.

[4] On November 17, 2017, the Company completed an offering of $450.0 million 3.95% Guaranteed Notes due 2027 (the “2027 Notes) and reopened the 3.95% Guaranteed Notes due 2023 (the “2023 Notes”) for an additional $100.0 million. On November 17, 2017, the Company redeemed the remaining outstanding balance of the 4.95% Guaranteed Notes due 2018 (the “2018 Notes”) ahead of its scheduled maturity. See below for further discussion.

[5] On September 30, 2017, the interest rate hedge contract for the Preferred Trust I expired. Subsequent to this expiration, the debt bears interest at a variable rate of LIBOR + 1.25%.