Derivative Instruments - Offsetting Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Total Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	$ 4,828	$ 29,040
Gross Amounts Offset	(4,776)	(18,475)
Net Assets	52	10,565
Cash Collateral Offset	0	0
Net Amount Presented	52	10,565
Total Current Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	4,247	19,383
Gross Amounts Offset	(4,195)	(12,094)
Net Assets	52	7,289
Cash Collateral Offset	0	0
Net Amount Presented	52	7,289
Non-trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	4,213	18,649
Gross Amounts Offset	(4,193)	(12,000)
Net Assets	20	6,649
Cash Collateral Offset	0	0
Net Amount Presented	20	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	581	9,657
Gross Amounts Offset	(581)	(6,381)
Net Assets	0	3,276
Cash Collateral Offset	0	0
Net Amount Presented	0	3,276
Non-trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	581	9,657
Gross Amounts Offset	(581)	(6,381)
Net Assets	0	3,276
Cash Collateral Offset	0	0
Net Amount Presented	0	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