Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis by Level (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent payment arrangement	$ (3,000)	$ (4,600)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	12,121	34,500
Derivative liabilities	(6,459)	(2,129)
Contingent payment arrangement	(2,980)	(4,650)
Fair Value, Measurements, Recurring | Non-trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	12,096	34,044
Derivative liabilities	(6,065)	(1,337)
Fair Value, Measurements, Recurring | Trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	25	456
Derivative liabilities	(394)	(792)
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	178	158
Derivative liabilities	(394)	(942)
Contingent payment arrangement	0	0
Fair Value, Measurements, Recurring | Level 1 | Non-trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	178	158
Derivative liabilities	(16)	(387)
Fair Value, Measurements, Recurring | Level 1 | Trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	(378)	(555)
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	11,943	34,342
Derivative liabilities	(6,065)	(1,187)
Contingent payment arrangement	0	0
Fair Value, Measurements, Recurring | Level 2 | Non-trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	11,918	33,886
Derivative liabilities	(6,049)	(950)
Fair Value, Measurements, Recurring | Level 2 | Trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	25	456
Derivative liabilities	(16)	(237)
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Contingent payment arrangement	(2,980)	(4,650)
Fair Value, Measurements, Recurring | Level 3 | Non-trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 3 | Trading Commodity Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	$ 0	$ 0