Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Book value	$ 3,146,611	$ 2,914,765
Fair value	3,129,696	3,081,932
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	543,400
Payments/Payoffs	(300,876)
Adjustments to fair value	(194,760)
Long-term Debt, Gross	3,176,689	2,934,187
Debt Issuance Costs, Net	30,078	19,422
Fixed Rate Secured Debt [Member]
Book value	49,789	32,287
Fair value	41,981	34,547
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	18,400
Payments/Payoffs	(876)
Adjustments to fair value	(10,090)
Variable Rate Secured Debt [Member]
Book value	1,900	1,900
Fair value	1,900	1,900
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,925,000	2,900,000
Fair value	2,885,815	3,045,485
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	325,000
Payments/Payoffs	(300,000)
Adjustments to fair value	(184,670)
Unsecured Line of Credit DRLP [Member]
Book value	200,000	0
Fair value	200,000	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	200,000
Payments/Payoffs	0
Adjustments to fair value	$ 0

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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