Acquisition of Shoal Creek Mine Pro Forma Results (Details) - Successor - Shoal Creek - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2018
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenue	$ 4,506.2	$ 6,008.4
Income from continuing operations, net of income taxes	$ 783.3	$ 826.6
Basic earnings per share from continuing operations	$ 4.37	$ 5.84
Diluted earnings per share from continuing operations	$ 4.33	$ 5.75

The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)-(3)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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Amount after tax of pro forma income from continuing operations as if the business combination had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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