Derivatives and Fair Value Measurements - Financial Instruments Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Equity securities	$ 4.0	$ 4.0
Total net financial assets	12.1	9.9
Level 1
Derivative [Line Items]
Equity securities	0.0	0.0
Total net financial assets	0.0	0.0
Level 2
Derivative [Line Items]
Equity securities	0.0	0.0
Total net financial assets	8.1	5.9
Level 3
Derivative [Line Items]
Equity securities	4.0	4.0
Total net financial assets	4.0	4.0
Foreign currency option contracts
Derivative [Line Items]
Derivative assets (liabilities), at fair value	6.1	1.1
Foreign currency option contracts \| Level 1
Derivative [Line Items]
Derivative assets (liabilities), at fair value	0.0	0.0
Foreign currency option contracts \| Level 2
Derivative [Line Items]
Derivative assets (liabilities), at fair value	6.1	1.1
Foreign currency option contracts \| Level 3
Derivative [Line Items]
Derivative assets (liabilities), at fair value	0.0	0.0
Coal contracts related to forecasted sales
Derivative [Line Items]
Derivative assets (liabilities), at fair value	10.2	21.2
Coal contracts related to forecasted sales \| Level 1
Derivative [Line Items]
Derivative assets (liabilities), at fair value	0.0	0.0
Coal contracts related to forecasted sales \| Level 2
Derivative [Line Items]
Derivative assets (liabilities), at fair value	10.2	21.2
Coal contracts related to forecasted sales \| Level 3
Derivative [Line Items]
Derivative assets (liabilities), at fair value	0.0	0.0
Coal trading contracts
Derivative [Line Items]
Derivative assets (liabilities), at fair value	(8.2)	(16.4)
Coal trading contracts \| Level 1
Derivative [Line Items]
Derivative assets (liabilities), at fair value	0.0	0.0
Coal trading contracts \| Level 2
Derivative [Line Items]
Derivative assets (liabilities), at fair value	(8.2)	(16.4)
Coal trading contracts \| Level 3
Derivative [Line Items]
Derivative assets (liabilities), at fair value	$ 0.0	$ 0.0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958

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+ References

No definition available.

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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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