Fair Value Measurements - Classification of Outstanding Derivative Instruments and Fair Value Hierarchy (Details) - Fair Value, Recurring $ in Thousands	Sep. 30, 2020 USD ($)
Current assets:
Natural gas derivative instruments	$ 2,181
Natural gas derivative instruments, Netting	(2,181)
Derivative Asset, Total	0
Current liabilities:
Natural gas derivative instruments	4,389
Natural gas derivative instruments, Netting	(2,181)
Derivative Liability, Total	2,208
Level 1
Current assets:
Natural gas derivative instruments	0
Current liabilities:
Natural gas derivative instruments	0
Level 2
Current assets:
Natural gas derivative instruments	2,181
Current liabilities:
Natural gas derivative instruments	4,389
Level 3
Current assets:
Natural gas derivative instruments	0
Current liabilities:
Natural gas derivative instruments	$ 0

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