Debt Obligations - Consolidated Debt Obligations Outstanding (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Consolidated debt obligations
Plus: premiums/(discounts), net	$ (5,329)
Less: deferred financing costs	(8,889)
Outstanding indebtedness	2,078,255	$ 2,028,046
Secured Debt
Consolidated debt obligations
Long-term debt, gross	317,363	323,032
Plus: premiums/(discounts), net	(1,477)	(1,759)
Less: deferred financing costs	(283)	(404)
Total mortgage indebtedness	315,603	320,869
Unsecured Debt
Consolidated debt obligations
Long-term debt, gross	1,775,110	1,721,110
Plus: premiums/(discounts), net	(3,852)	(4,096)
Less: deferred financing costs	(8,606)	(9,837)
Total unsecured indebtedness	1,762,652	1,707,177
Unsecured Debt \| $600 million Unsecured Credit Facility
Consolidated debt obligations
Long-term debt, gross	146,500	92,500
Unsecured Debt \| Seven-Year Term Loan - Swapped to fixed
Consolidated debt obligations
Long-term debt, gross	$ 250,000	250,000
Effective interest rate	2.87%
Unsecured Debt \| $350.0M 3.95% Guaranteed Notes due 2023
Consolidated debt obligations
Long-term debt, gross	$ 350,000	350,000
Effective interest rate	3.87%
Unsecured Debt \| $250.0M 4.10% Guaranteed Notes due 2024
Consolidated debt obligations
Long-term debt, gross	$ 250,000	250,000
Effective interest rate	4.33%
Unsecured Debt \| $450.0M 3.95% Guaranteed Notes due 2027
Consolidated debt obligations
Long-term debt, gross	$ 450,000	450,000
Effective interest rate	4.03%
Unsecured Debt \| $250.0M 4.55% Guaranteed Notes due 2029
Consolidated debt obligations
Long-term debt, gross	$ 250,000	250,000
Effective interest rate	4.60%
Unsecured Debt \| Indenture IA (Preferred Trust I)
Consolidated debt obligations
Long-term debt, gross	$ 27,062	27,062
Unsecured Debt \| Indenture IB (Preferred Trust I) - Swapped to fixed
Consolidated debt obligations
Long-term debt, gross	$ 25,774	25,774
Effective interest rate	3.30%
Unsecured Debt \| Indenture II (Preferred Trust II) - Swapped to fixed
Consolidated debt obligations
Long-term debt, gross	$ 25,774	25,774
Effective interest rate	3.09%
LIBOR \| Unsecured Debt \| $600 million Unsecured Credit Facility
Consolidated debt obligations
Spread on variable rate	1.10%
LIBOR \| Unsecured Debt \| Indenture IA (Preferred Trust I)
Consolidated debt obligations
Spread on variable rate	1.25%
Two Logan Square \| Secured Debt
Consolidated debt obligations
Long-term debt, gross	$ 81,535	82,805
Effective interest rate	3.98%
Four Tower Bridge \| Secured Debt
Consolidated debt obligations
Long-term debt, gross	$ 9,351	9,526
Effective interest rate	4.50%
One Commerce Square \| Secured Debt
Consolidated debt obligations
Long-term debt, gross	$ 117,486	120,183
Effective interest rate	3.64%
Two Commerce Square \| Secured Debt
Consolidated debt obligations
Long-term debt, gross	$ 108,991	$ 110,518
Effective interest rate	4.51%

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Secured debt net of premiums discount.

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

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