Accounting for Derivative Instruments - Offsetting Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Commodity Contract
Offsetting Liabilities [Line Items]
Gross Liabilities	$ (7,693)	$ (9,401)
Gross Amounts Offset	4,049	7,550
Net Liabilities	(3,644)	(1,851)
Cash Collateral Offset	360	0
Net Amount Presented	(3,284)	(1,851)
Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(7,636)	(9,275)
Gross Amounts Offset	3,995	7,442
Net Liabilities	(3,641)	(1,833)
Cash Collateral Offset	360	0
Net Amount Presented	(3,281)	(1,833)
Non-trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(6,545)	(8,289)
Gross Amounts Offset	3,414	6,898
Net Liabilities	(3,131)	(1,391)
Cash Collateral Offset	360	0
Net Amount Presented	(2,771)	(1,391)
Trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(1,091)	(986)
Gross Amounts Offset	581	544
Net Liabilities	(510)	(442)
Cash Collateral Offset	0	0
Net Amount Presented	(510)	(442)
Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(57)	(126)
Gross Amounts Offset	54	108
Net Liabilities	(3)	(18)
Cash Collateral Offset	0	0
Net Amount Presented	(3)	(18)
Non-trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(57)	(120)
Gross Amounts Offset	54	94
Net Liabilities	(3)	(26)
Cash Collateral Offset	0	0
Net Amount Presented	(3)	(26)
Trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities		(6)
Gross Amounts Offset		14
Net Liabilities		8
Cash Collateral Offset		0
Net Amount Presented		$ 8