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Derivatives and Fair Value Measurements - Pre-Tax Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Total gain (loss) recognized in income | $ (11.6) | $ 63.6 | $ 103.7
Gain realized in income on derivatives | 15.8 | 11.8 | 83.7
Unrealized gain (loss) recognized in income on derivatives | (27.4) | 51.8 | 20.0
Foreign currency option contracts |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Total gain (loss) recognized in income | 12.9 | (3.7) | (9.1)
Gain realized in income on derivatives | 5.8 | (4.9) | (8.4)
Unrealized gain (loss) recognized in income on derivatives | 7.1 | 1.2 | (0.7)
Coal contracts related to forecasted sales |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Total gain (loss) recognized in income | (23.8) | 67.6 | 115.7
Gain realized in income on derivatives | 5.8 | 25.4 | 97.4
Unrealized gain (loss) recognized in income on derivatives | (29.6) | 42.2 | 18.3
Coal trading contracts |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Total gain (loss) recognized in income | (0.7) | (0.3) | (2.9)
Gain realized in income on derivatives | 4.2 | (8.7) | (5.3)
Unrealized gain (loss) recognized in income on derivatives | $ (4.9) | $ 8.4 | $ 2.4

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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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- Definition Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e689-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + Details
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