Derivative Instruments and Fair Value Measurements - Use of Derivative Instruments (Details) - Commodity Contract - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenues – marketing
Derivatives, Fair Value [Line Items]
Gain (loss) on derivative	$ (20)	$ 1
Current Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	274		$ 162
Liability derivatives	0		0
Less counterparty offsets	0		0
As reported fair value contracts	274		162
Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	0		0
Liability derivatives	0		0
Less counterparty offsets	0		0
As reported fair value contracts	0		0
Current Liabilities
Derivatives, Fair Value [Line Items]
Asset derivatives	0		0
Liability derivatives	270		139
Less counterparty offsets	0		0
As reported fair value contracts	270		139
Other Liabilities
Derivatives, Fair Value [Line Items]
Asset derivatives	0		0
Liability derivatives	0		0
Less counterparty offsets	0		0
As reported fair value contracts	$ 0		$ 0