Commodity Derivative Financial Instruments - Summary of Fair Value and Classification of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Gross Fair Value, Assets	$ 12,105	$ 3,879
Effect of Counterparty Netting, Assets	(11,648)	(3,879)
Net Carrying Value on Balance Sheet, Assets	457	0
Gross Fair Value, Liabilities	17,133	20,598
Effect of Counterparty Netting, Liabilities	(11,648)	(3,879)
Net Carrying Value on Balance Sheet, Liabilities	5,485	16,719
Commodity Derivative Assets, Current
Derivatives Fair Value [Line Items]
Gross Fair Value, Assets	10,713	3,879
Effect of Counterparty Netting, Assets	(10,619)	(3,879)
Net Carrying Value on Balance Sheet, Assets	94	0
Deferred Charges and Other Long-term Assets
Derivatives Fair Value [Line Items]
Gross Fair Value, Assets	1,392	0
Effect of Counterparty Netting, Assets	(1,029)	0
Net Carrying Value on Balance Sheet, Assets	363	0
Commodity Derivative Liabilities, Current
Derivatives Fair Value [Line Items]
Gross Fair Value, Liabilities	14,841	20,116
Effect of Counterparty Netting, Liabilities	(10,619)	(3,879)
Net Carrying Value on Balance Sheet, Liabilities	4,222	16,237
Commodity Derivative Liabilities, Long-term
Derivatives Fair Value [Line Items]
Gross Fair Value, Liabilities	2,292	482
Effect of Counterparty Netting, Liabilities	(1,029)	0
Net Carrying Value on Balance Sheet, Liabilities	$ 1,263	$ 482