Derivative Instruments and Fair Value Measurements - Use of Derivative Instruments (Details) - Commodity Contract - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenues – marketing
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative	$ 1	$ 280	$ 2	$ 700
Current Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	538		538		$ 166
Liability derivatives	0		0		0
Less counterparty offsets	0		0		0
As reported fair value contracts	538		538		166
Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	0		0		0
Liability derivatives	0		0		0
Less counterparty offsets	0		0		0
As reported fair value contracts	0		0		0
Current Liabilities
Derivatives, Fair Value [Line Items]
Asset derivatives	0		0		0
Liability derivatives	514		514		145
Less counterparty offsets	0		0		0
As reported fair value contracts	514		514		145
Other Liabilities
Derivatives, Fair Value [Line Items]
Asset derivatives	0		0		0
Liability derivatives	0		0		0
Less counterparty offsets	0		0		0
As reported fair value contracts	$ 0		$ 0		$ 0