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 |