Fair Value Measurements - Reconciliation of Energy Derivative Contract Measured at Fair Value (Detail) - Energy Related Derivative [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balances, beginning of period	$ 12,198	$ 33,895	$ 26,872	$ 40,916
Total gain (loss) included in electricity sales	1,714	83	(2,356)	4,144
Settlements	(2,032)	(3,196)	(12,636)	(14,278)
Balances, end of period	$ 11,880	$ 30,782	$ 11,880	$ 30,782