Derivative Instruments (Detail) - Commodity Contract	12 Months Ended
Dec. 31, 2017
Minimum
Derivative [Line Items]
Length of time hedged in commodity hedge	24 months
Maximum
Derivative [Line Items]
Length of time hedged in commodity hedge	1 month

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