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
	Mar. 31, 2016	Mar. 31, 2015
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Assets, Beginning balance	$ 1,154	$ 2,779
Assets, Total net realized and unrealized gains (losses) included in operating results	60	(30)
Assets, Sales		(33)
Assets, Ending balance	1,214	2,716
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	60
Liabilities, Beginning balance	(1,154)	(2,779)
Liabilities, Total net realized and unrealized gains (losses) included in operating results	(60)	30
Liabilities, Settlements		33
Liabilities, Ending balance	(1,214)	$ (2,716)
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	$ (60)