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Derivative Instruments - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017
Commodity Futures Contracts |
Trading Activity, Gains and Losses, Net [Line Items] |
Reclassification period for effective portion of contract, in years | 1 year

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- Definition Reclassification period for effective portion of contract. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition 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. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |