Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Book value	$ 2,131,880	$ 2,908,477
Fair value	2,248,750	3,034,065
Payments/Payoffs	(780,302)
Adjustments to fair value	(5,013)
Long-term Debt, Gross	2,151,162	2,931,529
Debt Issuance Costs, Net	19,282	23,052
Fixed Rate Secured Debt [Member]
Book value	310,975	381,894
Fair value	332,025	415,231
Payments/Payoffs	(70,854)
Adjustments to fair value	(12,352)
Variable Rate Secured Debt [Member]
Book value	2,500	2,800
Fair value	2,500	2,800
Payments/Payoffs	(300)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	1,832,687	2,498,835
Fair value	1,909,225	2,568,034
Payments/Payoffs	(666,148)
Adjustments to fair value	7,339
Unsecured Line of Credit DRLP [Member]
Book value	5,000	48,000
Fair value	5,000	$ 48,000
Payments/Payoffs	(43,000)
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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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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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