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, 2017	Mar. 31, 2016
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Assets, Beginning balance		$ 1,154
Assets, Total net realized and unrealized gains (losses) included in operating results	$ 867	60
Assets, Ending balance	867	1,214
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	867	60
Liabilities, Beginning balance	(2,557)	(1,154)
Liabilities, Total net realized and unrealized gains (losses) included in operating results	(1,158)	(60)
Liabilities, Ending balance	(3,715)	(1,214)
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	$ (1,158)	$ (60)