Derivative Instruments - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Total Derivative Liabilities
Offsetting Liabilities [Line Items]
Gross Liabilities	$ (48,890)	$ (22,088)
Gross Amounts Offset	17,714	15,504
Net Liabilities	(31,176)	(6,584)
Cash Collateral Offset	750	0
Net Amount Presented	(30,426)	(6,584)
Total Current Derivative Liabilities
Offsetting Liabilities [Line Items]
Gross Liabilities	(42,350)	(21,882)
Gross Amounts Offset	15,752	15,404
Net Liabilities	(26,598)	(6,478)
Cash Collateral Offset	750	0
Net Amount Presented	(25,848)	(6,478)
Non-trading commodity derivatives
Offsetting Liabilities [Line Items]
Gross Liabilities	(42,293)	(21,391)
Gross Amounts Offset	15,695	15,385
Net Liabilities	(26,598)	(6,006)
Cash Collateral Offset	750	0
Net Amount Presented	(25,848)	(6,006)
Trading commodity derivatives
Offsetting Liabilities [Line Items]
Gross Liabilities	(57)	(491)
Gross Amounts Offset	57	19
Net Liabilities	0	(472)
Cash Collateral Offset	0	0
Net Amount Presented	0	(472)
Total Non-current Derivative Assets
Offsetting Liabilities [Line Items]
Gross Liabilities	(6,540)	(206)
Gross Amounts Offset	1,962	100
Net Liabilities	(4,578)	(106)
Cash Collateral Offset	0	0
Net Amount Presented	(4,578)	(106)
Non-trading commodity derivatives
Offsetting Liabilities [Line Items]
Gross Liabilities	(6,527)	(71)
Gross Amounts Offset	1,949	40
Net Liabilities	(4,578)	(31)
Cash Collateral Offset	0	0
Net Amount Presented	(4,578)	(31)
Trading commodity derivatives
Offsetting Liabilities [Line Items]
Gross Liabilities	(13)	(135)
Gross Amounts Offset	13	60
Net Liabilities	0	(75)
Cash Collateral Offset	0	0
Net Amount Presented	$ 0	$ (75)