Schedule of Realized Losses on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2015
Derivative Instruments Gain Loss [Line Items]
Losses on derivative financial instruments	$ 1.5	$ 2.2
Metal
Derivative Instruments Gain Loss [Line Items]
Losses on derivative financial instruments	$ 1.5	$ 2.2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DerivativeInstrumentsGainLossLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration