Consolidated Debt Obligations Abstract.

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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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- Definition

Face (par) amount of debt instrument at time of issuance.

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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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Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

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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The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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DEBT OBLIGATIONS - Consolidated Debt Obligations Outstanding (Details) - USD ($) $ in Thousands					12 Months Ended
	Nov. 10, 2020	Oct. 21, 2020	Dec. 13, 2018	Dec. 12, 2018	Dec. 31, 2020	Dec. 31, 2019
Consolidated debt obligations
Total Debt Obligations					$ 1,830,595	$ 2,144,418
Four Tower Bridge
Consolidated debt obligations
Repayments of long-term debt	$ 9,100
Secured Debt
Consolidated debt obligations
Long-term debt, gross					0	315,437
Plus: premiums/(discounts), net					0	(1,383)
Net deferred financing costs					0	(242)
Total Debt Obligations					0	313,812
Secured Debt \| Two Logan Square
Consolidated debt obligations
Long-term debt, gross					$ 0	81,103
Effective interest rate					3.98%
Repayments of long-term debt		$ 79,800
Secured Debt \| Four Tower Bridge
Consolidated debt obligations
Long-term debt, gross					$ 0	9,291
Effective interest rate					4.50%
Secured Debt \| One Commerce Square
Consolidated debt obligations
Long-term debt, gross					$ 0	116,571
Effective interest rate					3.64%
Secured Debt \| Two Commerce Square
Consolidated debt obligations
Long-term debt, gross					$ 0	108,472
Effective interest rate					4.51%
Unsecured Debt
Consolidated debt obligations
Long-term debt, gross					$ 1,828,610	1,828,610
Plus: premiums/(discounts), net					10,137	12,090
Net deferred financing costs					(8,152)	(10,094)
Total Debt Obligations					1,830,595	1,830,606
Unsecured Debt \| $600 million Unsecured Credit Facility
Consolidated debt obligations
Debt instrument, face amount					600,000
Long-term debt, gross					$ 0	0
Unsecured Debt \| Seven-Year Term Loan - Swapped to fixed
Consolidated debt obligations
Debt instrument, term			7 years		7 years
Debt instrument, interest rate, stated percentage			2.87%
Long-term debt, gross			$ 250,000		$ 250,000	250,000
Effective interest rate					2.87%
Unsecured Debt \| $350.0M 3.95% Guaranteed Notes due 2023
Consolidated debt obligations
Debt instrument, face amount					$ 350,000
Debt instrument, interest rate, stated percentage					3.95%
Long-term debt, gross					$ 350,000	350,000
Effective interest rate					3.87%
Unsecured Debt \| $350.0M 4.10% Guaranteed Notes due 2024
Consolidated debt obligations
Debt instrument, face amount					$ 350,000
Debt instrument, interest rate, stated percentage					4.10%
Long-term debt, gross					$ 350,000	350,000
Effective interest rate					3.78%
Unsecured Debt \| $450.0M 3.95% Guaranteed Notes due 2027
Consolidated debt obligations
Debt instrument, face amount					$ 450,000
Debt instrument, interest rate, stated percentage					3.95%
Long-term debt, gross					$ 450,000	450,000
Effective interest rate					4.03%
Unsecured Debt \| $350.0M 4.55% Guaranteed Notes due 2029
Consolidated debt obligations
Debt instrument, face amount					$ 350,000
Debt instrument, interest rate, stated percentage					4.55%
Long-term debt, gross					$ 350,000	350,000
Effective interest rate					4.30%
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)
Consolidated debt obligations
Long-term debt, gross					$ 25,774	$ 25,774
LIBOR \| Unsecured Debt \| $600 million Unsecured Credit Facility
Consolidated debt obligations
Variable interest rate					1.10%
LIBOR \| Unsecured Debt \| Seven-Year Term Loan - Swapped to fixed
Consolidated debt obligations
Variable interest rate			1.25%	1.80%
LIBOR \| Unsecured Debt \| Indenture IA (Preferred Trust I)
Consolidated debt obligations
Variable interest rate					1.25%
LIBOR \| Unsecured Debt \| Indenture II (Preferred Trust II)
Consolidated debt obligations
Variable interest rate					1.25%