Commodity Derivative Financial Instruments - Summary of Fair Value and Classification of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivatives Fair Value [Line Items]
Gross fair value amount	$ 34,891	$ 50,264
Effect of Counterparty Netting, Assets	(3,478)	(2,226)
Total net carrying value on Balance Sheet, assets	31,413	48,038
Gross Fair Value, Liabilities	3,523	2,226
Effect of Counterparty Netting, Liabilities	(3,478)	(2,226)
Total net carrying value on Balance Sheet, liabilities	45	0
Commodity derivative assets
Derivatives Fair Value [Line Items]
Gross fair value amount	26,959	38,746
Effect of Counterparty Netting, Assets	(2,518)	(776)
Total net carrying value on Balance Sheet, assets	24,441	37,970
Deferred charges and other long-term assets
Derivatives Fair Value [Line Items]
Gross fair value amount	7,932	11,518
Effect of Counterparty Netting, Assets	(960)	(1,450)
Total net carrying value on Balance Sheet, assets	6,972	10,068
Commodity derivative liabilities
Derivatives Fair Value [Line Items]
Gross Fair Value, Liabilities	2,518	776
Effect of Counterparty Netting, Liabilities	(2,518)	(776)
Total net carrying value on Balance Sheet, liabilities	0	0
Commodity derivative liabilities
Derivatives Fair Value [Line Items]
Gross Fair Value, Liabilities	1,005	1,450
Effect of Counterparty Netting, Liabilities	(960)	(1,450)
Total net carrying value on Balance Sheet, liabilities	$ 45	$ 0

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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