Divestitures (Narrative) (Details) $ in Millions		12 Months Ended
Jun. 27, 2013 mining_complex	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Discontinued Operations and Disposal Groups [Abstract]
Number of longwall mining complexes \| mining_complex	2
Proceeds from disposals and divestitures		$ 46.7	$ 422.7
Pre-tax gain on divestiture		$ 17.8

The gain (loss) resulting from the sale of a disposal group that is not a discontinued operation. It is included in income from continuing operations before income taxes in the income statement.

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=8077374&loc=d3e2443-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration