Derivatives, Hedges, Financial Instruments and Carbon Credits - Reconciliation of Beginning and Ending Balances for Assets and Liabilities Measured at Fair Value on Recurring Basis (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
Assets
Beginning balance	$ 1,673	$ 312	$ 1,284	$ 91
Transfers into Level 3
Transfers out of Level 3
Total realized and unrealized gains (losses) included in earnings		313	1,925	546
Purchases
Issuances
Sales			(1,536)	(12)
Settlements
Ending balance	1,673	625	1,673	625
Total gains (losses) for the period included in earnings results attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date		313	1,004	534
Liabilities
Beginning balance	(1,673)	(312)	(1,284)	(91)
Transfers into Level 3
Transfers out of Level 3
Total realized and unrealized gains (losses) included in earnings		(313)	(1,634)	(546)
Purchases
Issuances
Sales
Settlements			1,245	12
Ending balance	(1,673)	(625)	(1,673)	(625)
Total gains (losses) for the period included in earnings results attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date		$ (313)	$ (1,004)	$ (534)