Accounting for Derivative Instruments - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Commodity Contract
Offsetting Liabilities [Line Items]
Gross Liabilities	$ (11,518)	$ (15,977)
Gross Amounts Offset	1,952	3,973
Net Liabilities	(9,566)	(12,004)
Cash Collateral Offset	210	0
Net Amount Presented	(9,356)	(12,004)
Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(10,587)	(15,186)
Gross Amounts Offset	1,552	3,660
Net Liabilities	(9,035)	(11,526)
Cash Collateral Offset	210	0
Net Amount Presented	(8,825)	(11,526)
Non-trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(10,385)	(14,911)
Gross Amounts Offset	1,478	3,562
Net Liabilities	(8,907)	(11,349)
Cash Collateral Offset	210	0
Net Amount Presented	(8,697)	(11,349)
Trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(202)	(275)
Gross Amounts Offset	74	98
Net Liabilities	(128)	(177)
Cash Collateral Offset	0	0
Net Amount Presented	(128)	(177)
Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(931)	(791)
Gross Amounts Offset	400	313
Net Liabilities	(531)	(478)
Cash Collateral Offset	0	0
Net Amount Presented	(531)	(478)
Non-trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(931)	(791)
Gross Amounts Offset	400	313
Net Liabilities	(531)	(478)
Cash Collateral Offset	0	0
Net Amount Presented	$ (531)	$ (478)