Debt Obligations - Consolidated Debt Obligations Outstanding (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Consolidated debt obligations
Plus: premiums/(discounts), net	$ 10,651
Less: deferred financing costs	(8,638)
Outstanding indebtedness	1,919,511	$ 2,144,418
Secured Debt
Consolidated debt obligations
Principal balance outstanding	88,888	315,437
Plus: premiums/(discounts), net	25	(1,383)
Less: deferred financing costs	0	(242)
Outstanding indebtedness	88,913	313,812
Secured Debt \| Two Logan Square
Consolidated debt obligations
Principal balance outstanding	$ 79,781	81,103
Effective interest rate	3.98%
Secured Debt \| Four Tower Bridge
Consolidated debt obligations
Principal balance outstanding	$ 9,107	9,291
Effective interest rate	4.50%
Secured Debt \| One Commerce Square
Consolidated debt obligations
Principal balance outstanding	$ 0	116,571
Effective interest rate	3.64%
Secured Debt \| Two Commerce Square
Consolidated debt obligations
Principal balance outstanding	$ 0	108,472
Effective interest rate	4.51%
Unsecured Debt
Consolidated debt obligations
Principal balance outstanding	$ 1,828,610	1,828,610
Plus: premiums/(discounts), net	10,626	12,090
Less: deferred financing costs	(8,638)	(10,094)
Outstanding indebtedness	1,830,598	1,830,606
Unsecured Debt \| Seven-Year Term Loan - Swapped to fixed
Consolidated debt obligations
Principal balance outstanding	$ 250,000	250,000
Effective interest rate	2.87%
Unsecured Debt \| $350.0M 3.95% Guaranteed Notes due 2023
Consolidated debt obligations
Principal balance outstanding	$ 350,000	350,000
Effective interest rate	3.87%
Unsecured Debt \| $350.0M 4.10% Guaranteed Notes due 2024
Consolidated debt obligations
Principal balance outstanding	$ 350,000	350,000
Effective interest rate	3.78%
Unsecured Debt \| $450.0M 3.95% Guaranteed Notes due 2027
Consolidated debt obligations
Principal balance outstanding	$ 450,000	450,000
Effective interest rate	4.03%
Unsecured Debt \| $350.0M 4.55% Guaranteed Notes due 2029
Consolidated debt obligations
Principal balance outstanding	$ 350,000	350,000
Effective interest rate	4.30%
Unsecured Debt \| Indenture IA (Preferred Trust I)
Consolidated debt obligations
Principal balance outstanding	$ 27,062	27,062
Unsecured Debt \| Indenture IA (Preferred Trust I) \| LIBOR
Consolidated debt obligations
Spread on variable rate	1.25%
Unsecured Debt \| Indenture IB (Preferred Trust I) - Swapped to fixed
Consolidated debt obligations
Principal balance outstanding	$ 25,774	25,774
Effective interest rate	3.30%
Unsecured Debt \| Indenture II (Preferred Trust II)
Consolidated debt obligations
Principal balance outstanding	$ 25,774	$ 25,774
Unsecured Debt \| Indenture II (Preferred Trust II) \| LIBOR
Consolidated debt obligations
Spread on variable rate	1.25%

Consolidated Debt Obligations Abstract.

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- Definition

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

+ References

No definition available.

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- Definition

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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- Definition

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

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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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