Inventories (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Inventories [Line Items]
Materials and supplies	$ 110.9	$ 115.9
Raw coal	72.2	75.9
Saleable coal	120.6	116.0
Total	$ 303.7	$ 307.8

Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a)(4),(5))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 330
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 5.BB)
-URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 5
-Section BB

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant