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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Assets, Beginning balance	$ 867	$ 1,214		$ 1,154
Assets, Total net realized and unrealized gains (losses) included in operating results			$ 867	60
Assets, Sales	(867)	(1,208)	(867)	(1,208)
Assets, Ending balance		6		6
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	(3,715)	(1,214)	(2,557)	(1,154)
Liabilities, Transfers into Level 3		(5,817)		(5,817)
Liabilities, Total net realized and unrealized gains (losses) included in operating results	(109)	4,209	(1,267)	4,149
Liabilities, Settlements	687	951	687	951
Liabilities, Ending balance	(3,137)	(1,871)	(3,137)	(1,871)
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	$ (289)	$ 3,952	$ (580)	$ 1,383