Fair Value Measurements - Reconciliation of Energy Derivative Contract Measured at Fair Value (Detail) - Energy derivative [Member] - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] | ||||
| Unrealized gains (losses) | $ (800) | $ 4,600 | $ (15,000) | $ 1,600 |
| Fair Value, Measurements, Recurring [Member] | ||||
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] | ||||
| Beginning balance | 63,683 | 64,475 | ||
| Net gains included in electricity sales | 2,326 | 7,599 | 1,659 | 16,666 |
| Settlements | (3,144) | (2,969) | (16,629) | (15,066) |
| Ending balance | $ 48,713 | $ 66,075 | $ 48,713 | $ 66,075 |