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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balances, beginning of period	$ 15,824	$ 38,559	$ 26,872	$ 40,916
Total gain (loss) included in electricity sales	1,482	403	(4,071)	4,061
Settlements	(5,108)	(5,067)	(10,603)	(11,082)
Balances, end of period	$ 12,198	$ 33,895	$ 12,198	$ 33,895