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Derivatives and Fair Value Measurements - Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total (loss) gain recognized in income | $ 48.5 | $ 54.6
(Loss) gain realized in income on derivatives | 4.8 | 16.0
Unrealized (loss) gain recognized in income on derivatives | 43.7 | 38.6
Foreign currency option contracts |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total (loss) gain recognized in income | (1.1) | (4.2)
(Loss) gain realized in income on derivatives | (1.3) | (2.4)
Unrealized (loss) gain recognized in income on derivatives | 0.2 | (1.8)
Coal contracts related to forecasted sales |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total (loss) gain recognized in income | 50.7 | 59.8
(Loss) gain realized in income on derivatives | 10.9 | 21.2
Unrealized (loss) gain recognized in income on derivatives | 39.8 | 38.6
Coal trading contracts |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total (loss) gain recognized in income | (1.1) | (1.0)
(Loss) gain realized in income on derivatives | (4.8) | (2.8)
Unrealized (loss) gain recognized in income on derivatives | $ 3.7 | $ 1.8

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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/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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