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Derivatives, Hedges, Financial Instruments and Carbon Credits - Net Gains (Losses) Included in Operating Results and Statement of Operations Classifications (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Total net gains (losses) included in operating results $ 341 $ (306) $ (3,111)
Cost of Sales [Member] | Commodities Contracts [Member] | Not Designated as Hedging Instrument [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Total net gains (losses) included in operating results   140 (3,376)
Cost of Sales [Member] | Foreign Exchange Contracts [Member] | Not Designated as Hedging Instrument [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Total net gains (losses) included in operating results   5 (72)
Other Income [Member] | Carbon Credits [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Total net gains (losses) included in operating results 2,031 1,514 3,663
Other Expense [Member] | Contractual Obligations Relating to Carbon Credits [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Total net gains (losses) included in operating results (1,587) (982) (2,759)
Non-operating Other Expense [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Non-operating other expense - embedded derivative $ (103) $ (983) (520)
Interest Expense [Member] | Interest Rate Contracts [Member] | Not Designated as Hedging Instrument [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Total net gains (losses) included in operating results     $ (47)