Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Book value	$ 2,767,582	$ 2,658,501
Fair value	2,935,336	2,662,401
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	175,000
Payments/Payoffs	(74,652)
Adjustments to fair value	172,587
Long-term Debt, Gross	2,785,071	2,684,801
Debt Issuance Costs, Net	17,489	26,300
Fixed Rate Secured Debt [Member]
Book value	33,171	77,601
Fair value	35,723	80,238
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(44,352)
Adjustments to fair value	(163)
Variable Rate Secured Debt [Member]
Book value	1,900	2,200
Fair value	1,900	2,200
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(300)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,750,000	2,575,000
Fair value	2,897,713	2,549,963
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	175,000
Payments/Payoffs	0
Adjustments to fair value	172,750
Unsecured Line of Credit DRLP [Member]
Book value	0	30,000
Fair value	0	$ 30,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(30,000)
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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