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	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Assets, Beginning balance	$ 6	$ 4,049	$ 1,154	$ 2,779
Assets, Total net realized and unrealized gains (losses) included in operating results	1,196		1,256	1,334
Assets, Sales	(1,202)	(3,862)	(2,410)	(3,926)
Assets, Ending balance		187		187
Total net gains (losses) for the period included in operating results attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date				177
Liabilities, Beginning balance	(1,871)	(4,049)	(1,154)	(2,779)
Liabilities, Transfers into Level 3			(5,817)
Liabilities, Total net realized and unrealized gains (losses) included in operating results	(3,395)	904	754	(430)
Liabilities, Settlements	2,661	2,958	3,612	3,022
Liabilities, Ending balance	(2,605)	$ (187)	(2,605)	(187)
Total net gains (losses) for the period included in operating results attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date	$ (2,199)		$ (816)	$ (177)