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Derivative - Net Gain (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	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)

X

- 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

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- Definition

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+ References

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