Derivative Financial Instruments - Summary of Outstanding Commodity Derivative Agreements (Detail) - Commodity Hedging Agreements [Member] - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Notional amount of outstanding commodity derivative agreements	$ 143,282,000	$ 59,564,000
Latest maturity date of outstanding commodity derivative agreements	2019-12	2018-12

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.

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