Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Commodity derivative contracts	$ 5,286	$ 5,582
Commodity derivative contracts		18,467
Recurring Measurement Basis
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments		5,072
Total Assets	5,286	6,382
Netting, assets	0	(4,272)
Netting, liabilities		(4,272)
Liabilities at Fair Value		14,195
Recurring Measurement Basis \| Commodity derivative contracts
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Netting, assets	0	(4,272)
Assets at Fair Value	5,286	1,310
Netting, liabilities		(4,272)
Liabilities at Fair Value		14,195
Recurring Measurement Basis \| Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments		5,072
Total Assets	0	5,072
Commodity derivative contracts		0
Recurring Measurement Basis \| Level 1 \| Commodity derivative contracts
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Commodity derivative contracts	0	0
Commodity derivative contracts		0
Recurring Measurement Basis \| Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments		0
Total Assets	5,286	5,582
Commodity derivative contracts		18,467
Recurring Measurement Basis \| Level 2 \| Commodity derivative contracts
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Commodity derivative contracts	5,286	5,582
Commodity derivative contracts		18,467
Recurring Measurement Basis \| Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments		0
Total Assets	0	0
Commodity derivative contracts		0
Recurring Measurement Basis \| Level 3 \| Commodity derivative contracts
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Commodity derivative contracts	$ 0	0
Commodity derivative contracts		$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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