Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Book value	$ 2,826,824	$ 2,658,501
Fair value	2,909,556	2,662,401
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	210,000
Payments/Payoffs	(42,665)
Adjustments to fair value	79,820
Long-term Debt, Gross	2,852,100	2,684,801
Debt Issuance Costs, Net	25,276	26,300
Fixed Rate Secured Debt [Member]
Book value	34,900	77,601
Fair value	37,193	80,238
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(42,665)
Adjustments to fair value	(380)
Variable Rate Secured Debt [Member]
Book value	2,200	2,200
Fair value	2,200	2,200
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	2,575,000	2,575,000
Fair value	2,630,163	2,549,963
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	0
Adjustments to fair value	80,200
Unsecured Line of Credit DRLP [Member]
Book value	240,000	30,000
Fair value	240,000	$ 30,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	210,000
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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