Accounting for Derivative Instruments - Offsetting Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Total Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	$ 52,797	$ 77,140
Gross Amounts Offset	(27,497)	(42,640)
Net Assets	25,300	34,500
Cash Collateral Offset	0	0
Net Amount Presented	25,300	34,500
Total Current Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	42,794	61,085
Gross Amounts Offset	(19,367)	(29,894)
Net Assets	23,427	31,191
Cash Collateral Offset	0	0
Net Amount Presented	23,427	31,191
Non-trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	42,794	60,167
Gross Amounts Offset	(19,367)	(29,432)
Net Assets	23,427	30,735
Cash Collateral Offset	0	0
Net Amount Presented	23,427	30,735
Trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	0	918
Gross Amounts Offset	0	(462)
Net Assets	0	456
Cash Collateral Offset	0	0
Net Amount Presented	0	456
Total Non-current Derivative Assets
Offsetting Assets [Line Items]
Gross Assets	10,003	16,055
Gross Amounts Offset	(8,130)	(12,746)
Net Assets	1,873	3,309
Cash Collateral Offset	0	0
Net Amount Presented	1,873	3,309
Non-trading commodity derivatives
Offsetting Assets [Line Items]
Gross Assets	10,003	16,055
Gross Amounts Offset	(8,130)	(12,746)
Net Assets	1,873	3,309
Cash Collateral Offset	0	0
Net Amount Presented	1,873	3,309
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