Schedule of Realized Losses on Derivative Financial Instruments (Detail) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($)
Derivative Instruments Gain Loss [Line Items]
Losses on derivative financial instruments	$ 3.4
Metal
Derivative Instruments Gain Loss [Line Items]
Losses on derivative financial instruments	2.9
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Losses on derivative financial instruments	$ 0.5

Amount of decrease in the fair value of derivatives recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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