Debt Obligations - Consolidated Debt Obligations Outstanding (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Consolidated debt obligations
Plus: premiums/(discounts), net	$ (5,679)
Less: deferred financing costs	(9,794)
Total Debt Obligations	2,094,797	$ 2,028,046
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	321,160	323,032
Plus: premiums/(discounts), net	(1,664)	(1,759)
Less: deferred financing costs	(364)	(404)
Total mortgage indebtedness	319,132	320,869
Secured Debt [Member] \| Two Logan Square [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 82,385	82,805
Effective interest rate	3.98%
Debt instrument maturity date	May 31, 2020
Secured Debt [Member] \| Four Tower Bridge [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 9,469	9,526
Effective interest rate	4.50%
Debt instrument maturity date	Feb. 28, 2021
Secured Debt [Member] \| One Commerce Square [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 119,292	120,183
Effective interest rate	3.64%
Debt instrument maturity date	Apr. 30, 2023
Secured Debt [Member] \| Two Commerce Square [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 110,014	110,518
Effective interest rate	4.51%
Debt instrument maturity date	Apr. 30, 2023
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 1,789,110	1,721,110
Plus: premiums/(discounts), net	(4,015)	(4,096)
Less: deferred financing costs	(9,430)	(9,837)
Total unsecured indebtedness	1,775,665	1,707,177
Unsecured Debt [Member] \| $600 million Unsecured Credit Facility [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 160,500	92,500
Debt instrument, description of variable rate basis	LIBOR + 1.10%
Spread on variable rate	1.10%
Debt instrument maturity date	Jul. 31, 2022
Unsecured Debt [Member] \| Seven Year Term Loan - Swapped to fixed [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 250,000	250,000
Effective interest rate	2.87%
Debt instrument maturity date	Oct. 31, 2022
Unsecured Debt [Member] \| $350M 3.95% Guaranteed Notes due 2023 [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 350,000	350,000
Effective interest rate	3.87%
Debt instrument maturity date	Feb. 28, 2023
Unsecured Debt [Member] \| $250.0M 4.10% Guaranteed Notes due 2024 [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 250,000	250,000
Effective interest rate	4.33%
Debt instrument maturity date	Oct. 31, 2024
Unsecured Debt [Member] \| $450.0M 3.95% Guaranteed Notes due 2027 [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 450,000	450,000
Effective interest rate	4.03%
Debt instrument maturity date	Nov. 30, 2027
Unsecured Debt [Member] \| $250M 4.55% Guaranteed Notes due 2029 [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 250,000	250,000
Effective interest rate	4.60%
Debt instrument maturity date	Oct. 31, 2029
Unsecured Debt [Member] \| Indenture IA (Preferred Trust I) [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 27,062	27,062
Debt instrument, description of variable rate basis	LIBOR + 1.25%
Spread on variable rate	1.25%
Debt instrument maturity date	Mar. 31, 2035
Unsecured Debt [Member] \| Indenture IB (Preferred Trust I) - Swapped to fixed [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 25,774	25,774
Effective interest rate	3.30%
Debt instrument maturity date	Apr. 30, 2035
Unsecured Debt [Member] \| Indenture II (Preferred Trust II) - Swapped to fixed [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 25,774	$ 25,774
Effective interest rate	3.09%
Debt instrument maturity date	Jul. 31, 2035

Consolidated Debt Obligations Abstract.

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Secured debt net of premiums discount.

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Total unsecured indebtedness.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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