Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	$ 52	$ 10,565
Total commodity derivative liabilities	(30,426)	(6,584)
Contingent payment arrangement	(1,328)	(1,328)
Non-trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	20	9,925
Total commodity derivative liabilities	(30,426)	(6,037)
Trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	32	640
Total commodity derivative liabilities	0	(547)
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	0	148
Total commodity derivative liabilities	(1,415)	(427)
Contingent payment arrangement	0	0
Level 1 | Non-trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	0	104
Total commodity derivative liabilities	(1,415)	(352)
Level 1 | Trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	0	44
Total commodity derivative liabilities	0	(75)
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	52	10,417
Total commodity derivative liabilities	(29,011)	(6,157)
Contingent payment arrangement	0	0
Level 2 | Non-trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	20	9,821
Total commodity derivative liabilities	(29,011)	(5,685)
Level 2 | Trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	32	596
Total commodity derivative liabilities	0	(472)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	0	0
Total commodity derivative liabilities	0	0
Contingent payment arrangement	(1,328)	(1,328)
Level 3 | Non-trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	0	0
Total commodity derivative liabilities	0	0
Level 3 | Trading commodity derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total commodity derivative assets	0	0
Total commodity derivative liabilities	$ 0	$ 0