Derivative Instruments - Gains and Losses from Derivative Instruments (Details) - USD ($) $ in Thousands |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2018 |
Jun. 30, 2017 |
Jun. 30, 2018 |
Jun. 30, 2017 |
|
| Derivative Instruments and Hedging Activities Disclosure [Abstract] | ||||
| Net gain (loss) on derivative instruments | $ 16,697 | $ 2,529 | $ 24,540 | $ 6,288 |