Derivative Instruments - Gains and Losses from Derivative Instruments (Details) - USD ($) $ in Thousands |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Derivative Instruments and Hedging Activities Disclosure [Abstract] | ||
| Realized gain (loss) on derivative instruments | $ 361 | $ (397) |
| Unrealized gain (loss) on derivative instruments | 7,482 | 4,156 |
| Net gain (loss) on derivative instruments | $ 7,843 | $ 3,759 |