Accounting for Derivative Instruments - Offsetting Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Commodity Contract
Offsetting Assets [Line Items]
Gross Assets	$ 3,272	$ 1,000
Gross Amounts Offset	(1,418)	(395)
Net Assets	1,854	605
Cash Collateral Offset	0	0
Net Amount Presented	1,854	605
Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	2,861	1,000
Gross Amounts Offset	(1,325)	(395)
Net Assets	1,536	605
Cash Collateral Offset	0	0
Net Amount Presented	1,536	605
Trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	952	411
Gross Amounts Offset	(13)	(6)
Net Assets	939	405
Cash Collateral Offset	0	0
Net Amount Presented	939	405
Not Designated as Hedging Instrument [Member] | Non-trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	1,909	589
Gross Amounts Offset	(1,312)	(389)
Net Assets	597	200
Cash Collateral Offset	0	0
Net Amount Presented	597	200
Not Designated as Hedging Instrument [Member] | Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	411	0
Gross Amounts Offset	(93)	0
Net Assets	318	0
Cash Collateral Offset	0	0
Net Amount Presented	318	0
Not Designated as Hedging Instrument [Member] | Non-trading Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	411	0
Gross Amounts Offset	(93)	0
Net Assets	318	0
Cash Collateral Offset	0	0
Net Amount Presented	$ 318	$ 0