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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Total net gains (losses) included in earnings:
Total net gains (losses) included in earnings	$ (142)	$ (25)	$ 2,345	$ (186)
Cost of sales [Member] | Commodities contracts [Member] | Not Designated as Hedging Instrument [Member]
Total net gains (losses) included in earnings:
Total net gains (losses) included in earnings	(105)	(113)	2,111	(275)
Other income [Member] | Carbon credits [Member]
Total net gains (losses) included in earnings:
Total net gains (losses) included in earnings		313	1,925	546
Other expense [Member] | Contractual obligations relating to carbon credits [Member]
Total net gains (losses) included in earnings:
Total net gains (losses) included in earnings		(313)	(1,634)	(546)
Interest expense [Member] | Interest rate contracts [Member] | Not Designated as Hedging Instrument [Member]
Total net gains (losses) included in earnings:
Total net gains (losses) included in earnings	$ (37)	$ 88	$ (57)	$ 89