Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2017	Dec. 31, 2016
Book value	$ 2,280,128	$ 2,908,477
Fair value	2,406,248	3,034,065
Payments/Payoffs	(630,976)
Adjustments to fair value	3,159
Long-term Debt, Gross	2,300,495	2,931,529
Debt Issuance Costs, Net	20,367	23,052
Fixed Rate Secured Debt [Member]
Book value	335,713	381,894
Fair value	359,928	415,231
Payments/Payoffs	(46,123)
Adjustments to fair value	(9,180)
Variable Rate Secured Debt [Member]
Book value	2,800	2,800
Fair value	2,800	2,800
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	1,961,982	2,498,835
Fair value	2,043,520	2,568,034
Payments/Payoffs	(536,853)
Adjustments to fair value	12,339
Unsecured Line of Credit DRLP [Member]
Book value	0	48,000
Fair value	0	$ 48,000
Payments/Payoffs	(48,000)
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
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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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