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Financial Instruments (Narrative) (Details) $ in Thousands	1 Months Ended	9 Months Ended
	Aug. 31, 2017 USD ($) a	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of acres | a	6
Proceeds from sale of property, plant, and equipment	$ 12,200
Promissory note received in sale of property, plant, and equipment	$ 5,700
Stated interest rate on promissory note	3.50%
Gain on sale of land	$ 3,300	$ 0	$ 3,307
Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured notes		$ 375,000		$ 375,000

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

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