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Derivative - Net Gain (Loss) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Derivative [Line Items] |  |  |
Derivative, gain (loss) on derivative, net | $ (17.3) | $ 12.0 | $ 6.3
Commodity Contract [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, gain (loss) on derivative, net | (16.3) | 12.9 | 6.6
Commodity Contract [Member] | Glencore [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, gain (loss) on derivative, net | (9.2) | 2.0 | (2.2)
Foreign Exchange Contract [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, gain (loss) on derivative, net | $ (1.0) | $ (0.9) | $ (0.3)

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959 + Details
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