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Derivatives - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
Derivatives - Narrative (Details) $ in Thousands	Jun. 30, 2018 USD ($) financialInstitution	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) financialInstitution	Jun. 30, 2017 USD ($)
Derivative Instruments And Hedging Activities Disclosure
Loss (gain) on derivative contracts	$ 30,104	$ (23,543)	$ 48,434	$ (57,726)
Cash paid on settlement of derivative contracts			$ 17,393	(2,706)
Number of counterparties to open derivative contracts \| financialInstitution	5		5
Commodity Derivatives
Derivative Instruments And Hedging Activities Disclosure
Loss (gain) on derivative contracts	$ 30,100	(23,500)	$ 48,400	(57,700)
Cash paid on settlement of derivative contracts	$ 11,300	$ (3,300)	$ 17,400	$ (2,700)