Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Book value	$ 2,417,429	$ 2,422,891
Fair value	2,423,023	2,518,801
Payments/Payoffs	(6,879)
Adjustments to fair value	(88,899)
Long-term Debt, Gross	2,437,087	2,444,005
Debt Issuance Costs, Net	19,658	21,114
Fixed Rate Secured Debt [Member]
Book value	303,867	309,463
Fair value	316,558	325,753
Payments/Payoffs	(5,557)
Adjustments to fair value	(3,638)
Variable Rate Secured Debt [Member]
Book value	2,500	2,500
Fair value	2,500	2,500
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,130,720	2,132,042
Fair value	2,103,965	$ 2,190,548
Payments/Payoffs	(1,322)
Adjustments to fair value	$ (85,261)

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
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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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