Segment Information Segment Results (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Reportable segment results
Revenues	$ 1,250.6	$ 1,462.7
Total Adjusted EBITDA	253.9	363.9
Net gain on disposals	1.5	30.6
Iffley Land Transaction
Reportable segment results
Net gain on disposals		20.6
Red Mountain Joint Venture
Reportable segment results
Net gain on disposals		7.1
Seaborne Thermal Mining \| Operating Segments
Reportable segment results
Revenues	251.0	201.4
Total Adjusted EBITDA	94.7	61.6
Seaborne Metallurgical Mining \| Operating Segments
Reportable segment results
Revenues	324.5	466.2
Total Adjusted EBITDA	85.8	166.4
Powder River Basin Mining \| Operating Segments
Reportable segment results
Revenues	287.3	389.3
Total Adjusted EBITDA	36.4	74.5
Midwestern U.S. Mining \| Operating Segments
Reportable segment results
Revenues	179.1	201.7
Total Adjusted EBITDA	33.3	31.2
Western U.S. Mining \| Operating Segments
Reportable segment results
Revenues	155.7	143.7
Total Adjusted EBITDA	42.6	32.0
Corporate and Other \| Corporate, Non-Segment
Reportable segment results
Revenues	53.0	60.4
Total Adjusted EBITDA	$ (38.9)	$ (1.8)

Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization.

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- Definition

Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2443-110228

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
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