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Financial Instruments (Narrative) (Details) $ in Millions | 1 Months Ended |  |
--- | --- | --- | ---
Aug. 31, 2017 USD ($) a | Jun. 30, 2018 USD ($) | 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 | $ 12.2 |  |
Promissory note received in sale of property, plant, and equipment | $ 5.7 |  |
Stated interest rate on promissory note | 3.50% |  |
Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Unsecured notes |  | $ 375.0 | $ 375.0

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