Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=51824106&loc=d3e2420-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Name:	us-gaap_AssetImpairmentCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Subsequent Event (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
Subsequent Event (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016
Subsequent Event [Line Items]
Asset impairment	$ 0.0	$ 0.0	$ 17.2	$ 900.8
Metropolitan mine [Member]
Subsequent Event [Line Items]
Cash consideration for asset held for sale	$ 200.0		$ 200.0
Metropolitan mine [Member] \| Scenario, Forecast [Member]
Subsequent Event [Line Items]
Asset impairment					$ 180.0