Accounting for Derivative Instruments - Offsetting Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commodity Contract
Offsetting Assets [Line Items]
Gross Assets	$ 35,023	$ 77,140
Gross Amounts Offset	(22,902)	(42,640)
Net Assets	12,121	34,500
Cash Collateral Offset	0	0
Net Amount Presented	12,121	34,500
Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	33,988	61,085
Gross Amounts Offset	(22,462)	(29,894)
Net Assets	11,526	31,191
Cash Collateral Offset	0	0
Net Amount Presented	11,526	31,191
Non-trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	33,897	60,167
Gross Amounts Offset	(22,396)	(29,432)
Net Assets	11,501	30,735
Cash Collateral Offset	0	0
Net Amount Presented	11,501	30,735
Trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	91	918
Gross Amounts Offset	(66)	(462)
Net Assets	25	456
Cash Collateral Offset	0	0
Net Amount Presented	25	456
Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	1,035	16,055
Gross Amounts Offset	(440)	(12,746)
Net Assets	595	3,309
Cash Collateral Offset	0	0
Net Amount Presented	595	3,309
Non-trading Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	1,035	16,055
Gross Amounts Offset	(440)	(12,746)
Net Assets	595	3,309
Cash Collateral Offset	0	0
Net Amount Presented	595	3,309
Trading Commodity Contract, Noncurrent
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