Derivatives and Fair Value Measurements Changes in Recurring Level 3 Net Financial Assets (Details) - Coal Trading - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Beginning of period	$ (1.1)	$ (0.7)	$ 10.0	$ 0.0
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.2	0.7	0.0	0.0
Total gains realized/unrealized:
Included in earnings	0.2	0.0	(9.0)	(1.7)
Purchases	0.0	0.0	3.0	10.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	1.7
End of period	$ (0.7)	$ 0.0	$ 4.0	$ 10.0

[Line Items] for Fair Value of Assets and Liabilities from Coal Trading Activities [Table]

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Gains or losses (realized/unrealized).

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sales of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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