Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Book value	$ 2,658,501	$ 2,422,891
Fair value	2,662,401	2,518,801
Debt Issuance Costs, Net	26,300	21,114
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	480,000
Long-term Debt, Gross	2,684,801	$ 2,444,005
Payoffs	(239,125)
Adjustments to fair value	$ (97,275)
Fixed Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	6.91%	7.43%
Book value	$ 77,601	$ 309,463
Fair value	80,238	$ 325,753
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(231,783)
Adjustments to fair value	$ (13,732)
Variable Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	1.72%	1.85%
Book value	$ 2,200	$ 2,500
Fair value	2,200	$ 2,500
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(300)
Adjustments to fair value	$ 0
Unsecured Debt [Member]
Debt, Weighted Average Interest Rate	3.92%	3.89%
Book value	$ 2,575,000	$ 2,132,042
Fair value	2,549,963	2,190,548
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	450,000
Payoffs	(7,042)
Adjustments to fair value	$ (83,543)
Unsecured Line of Credit DRLP [Member]
Debt, Weighted Average Interest Rate	3.39%
Book value	$ 30,000	0
Fair value	30,000	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	30,000
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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