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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 | 9 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | $ (23.0) | $ (5.7) | $ 66.5 | $ 8.5
Loss realized in income on derivatives | (7.3) | 17.6 | 10.6 | 44.0
Unrealized gain (loss) recognized in income on derivatives | (15.7) | (23.3) | 55.9 | (35.5)
Foreign currency option contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | (1.0) | (1.5) | (3.5) | (7.9)
Loss realized in income on derivatives | (1.3) | (1.8) | (3.7) | (6.5)
Unrealized gain (loss) recognized in income on derivatives | 0.3 | 0.3 | 0.2 | (1.4)
Coal contracts related to forecasted sales |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | (22.7) | (4.5) | 70.5 | 18.8
Loss realized in income on derivatives | (4.7) | 22.3 | 26.3 | 55.1
Unrealized gain (loss) recognized in income on derivatives | (18.0) | (26.8) | 44.2 | (36.3)
Coal trading contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | 0.7 | 0.3 | (0.5) | (2.4)
Loss realized in income on derivatives | (1.3) | (2.9) | (12.0) | (4.6)
Unrealized gain (loss) recognized in income on derivatives | $ 2.0 | $ 3.2 | $ 11.5 | $ 2.2

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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=109980867&loc=SL5618551-113959 + 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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4CC -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109998890-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + Details
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