Derivatives, Hedges, Financial Instruments and Carbon Credits - Net Gains (Losses) Included in Earnings and the Income Statement Classifications (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Total net gains (losses) included in operating results:
Total net gains (losses) included in operating results	$ (200)	$ (90)	$ 2,145	$ (276)
Cost of sales [Member] | Commodities contracts [Member] | Not Designated as Hedging Instrument [Member]
Total net gains (losses) included in operating results:
Total net gains (losses) included in operating results	(214)		1,897	(275)
Other income [Member] | Carbon credits [Member]
Total net gains (losses) included in operating results:
Total net gains (losses) included in operating results			1,925	546
Other expense [Member] | Contractual obligations relating to carbon credits [Member]
Total net gains (losses) included in operating results:
Total net gains (losses) included in operating results			(1,634)	(546)
Interest expense [Member] | Interest rate contracts [Member] | Not Designated as Hedging Instrument [Member]
Total net gains (losses) included in operating results:
Total net gains (losses) included in operating results	$ 14	$ (90)	$ (43)	$ (1)