Derivative Instruments - Offsetting Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Total Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	$ 13,207	$ 29,040
Gross Amounts Offset	(11,970)	(18,475)
Net Assets	1,237	10,565
Cash Collateral Offset	0	0
Net Amount Presented	1,237	10,565
Total Current Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	12,719	19,383
Gross Amounts Offset	(11,576)	(12,094)
Net Assets	1,143	7,289
Cash Collateral Offset	0	0
Net Amount Presented	1,143	7,289
Non-trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	12,685	18,649
Gross Amounts Offset	(11,574)	(12,000)
Net Assets	1,111	6,649
Cash Collateral Offset	0	0
Net Amount Presented	1,111	6,649
Trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	34	734
Gross Amounts Offset	(2)	(94)
Net Assets	32	640
Cash Collateral Offset	0	0
Net Amount Presented	32	640
Total Non-current Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	488	9,657
Gross Amounts Offset	(394)	(6,381)
Net Assets	94	3,276
Cash Collateral Offset	0	0
Net Amount Presented	94	3,276
Non-trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	488	9,657
Gross Amounts Offset	(394)	(6,381)
Net Assets	94	3,276
Cash Collateral Offset	0	0
Net Amount Presented	94	3,276
Trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	0	0
Gross Amounts Offset	0	0
Net Assets	0	0
Cash Collateral Offset	0	0
Net Amount Presented	$ 0	$ 0