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	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Assets, Beginning balance	$ 1,154	$ 2,779	$ 1,284
Assets, Total net realized and unrealized gains (losses) included in operating results	1,256	2,351	3,089
Assets, Sales	(2,410)	(3,976)	(1,594)
Assets, Ending balance		1,154	2,779
Total 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		1,143	2,110
Liabilities, Beginning balance	(1,154)	(2,779)	(1,284)
Liabilities, Transfers into Level 3	(5,817)
Liabilities, Total net realized and unrealized gains (losses) included in operating results	802	(1,447)	(2,799)
Liabilities, Settlements	3,612	3,072	1,304
Liabilities, Ending balance	(2,557)	(1,154)	(2,779)
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	$ (983)	$ (1,143)	$ (2,110)