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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balances, beginning of period	$ 38,559	$ 58,858	$ 40,916	$ 63,683
Total gain (loss) included in electricity sales	403	(2,575)	4,061	(667)
Settlements	(5,067)	(6,752)	11,082	13,485
Balances, end of period	$ 33,895	$ 49,531	$ 33,895	$ 49,531