Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Book value	$ 2,496,456	$ 2,422,891
Fair value	2,527,542	2,518,801
Issuances and Assumptions	75,000
Payments/Payoffs	(2,001)
Adjustments to fair value	(64,258)
Long-term Debt, Gross	2,516,985	2,444,005
Debt Issuance Costs, Net	20,529	21,114
Fixed Rate Secured Debt [Member]
Book value	308,099	309,463
Fair value	320,652	325,753
Issuances and Assumptions	0
Payments/Payoffs	(1,345)
Adjustments to fair value	(3,756)
Variable Rate Secured Debt [Member]
Book value	2,500	2,500
Fair value	2,500	2,500
Issuances and Assumptions	0
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,131,386	2,132,042
Fair value	2,129,390	2,190,548
Issuances and Assumptions	0
Payments/Payoffs	(656)
Adjustments to fair value	(60,502)
Unsecured Line of Credit DRLP [Member]
Book value	75,000	0
Fair value	75,000	$ 0
Issuances and Assumptions	75,000
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.

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