Accounting for Derivative Instruments - Offsetting Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Commodity Contract
Offsetting Assets [Line Items]
Gross Assets	$ 3,759	$ 28,145
Gross Amounts Offset	(2,802)	(16,718)
Net Assets	957	11,427
Cash Collateral Offset	0	0
Net Amount Presented	957	11,427
Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	2,424	20,271
Gross Amounts Offset	(1,589)	(11,927)
Net Assets	835	8,344
Cash Collateral Offset	0	0
Net Amount Presented	835	8,344
Trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	264	614
Gross Amounts Offset	(32)	(83)
Net Assets	232	531
Cash Collateral Offset	0	0
Net Amount Presented	232	531
Not Designated as Hedging Instrument | Non-trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	2,160	19,657
Gross Amounts Offset	(1,557)	(11,844)
Net Assets	603	7,813
Cash Collateral Offset	0	0
Net Amount Presented	603	7,813
Not Designated as Hedging Instrument | Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	1,335	7,874
Gross Amounts Offset	(1,213)	(4,791)
Net Assets	122	3,083
Cash Collateral Offset	0	0
Net Amount Presented	122	3,083
Not Designated as Hedging Instrument | Non-trading Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	1,029	7,874
Gross Amounts Offset	(1,145)	(4,791)
Net Assets	(116)	3,083
Cash Collateral Offset	0	0
Net Amount Presented	(116)	$ 3,083
Not Designated as Hedging Instrument | Trading Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	306
Gross Amounts Offset	(68)
Net Assets	238
Cash Collateral Offset	0
Net Amount Presented	$ 238