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Derivatives - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
Derivatives - Narrative (Details) $ in Thousands	Sep. 30, 2018 USD ($) institution	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) institution	Sep. 30, 2017 USD ($)
Derivative Instruments and Hedging Activities Disclosure
Loss (gain) on derivative contracts	$ 11,329	$ 11,702	$ 59,763	$ (46,024)
Cash payments, upon settlement of derivative contract			$ 29,025	(7,700)
Number of counterparties to open derivative contracts \| institution	5		5
Commodity Derivatives
Derivative Instruments and Hedging Activities Disclosure
Loss (gain) on derivative contracts	$ 11,300	11,700	$ 59,800	(46,000)
Cash payments, upon settlement of derivative contract	$ 11,600	$ (5,000)	$ 29,000	$ (7,700)