Accounting for Derivative Instruments - Offsetting Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commodity Contract
Offsetting Liabilities [Line Items]
Gross Liabilities	$ (15,977)	$ (9,401)
Gross Amounts Offset	3,973	7,550
Net Liabilities	(12,004)	(1,851)
Cash Collateral Offset	0	0
Net Amount Presented	(12,004)	(1,851)
Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(15,186)	(9,275)
Gross Amounts Offset	3,660	7,442
Net Liabilities	(11,526)	(1,833)
Cash Collateral Offset	0	0
Net Amount Presented	(11,526)	(1,833)
Non-trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(14,911)	(8,289)
Gross Amounts Offset	3,562	6,898
Net Liabilities	(11,349)	(1,391)
Cash Collateral Offset	0	0
Net Amount Presented	(11,349)	(1,391)
Trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(275)	(986)
Gross Amounts Offset	98	544
Net Liabilities	(177)	(442)
Cash Collateral Offset	0	0
Net Amount Presented	(177)	(442)
Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(791)	(126)
Gross Amounts Offset	313	108
Net Liabilities	(478)	(18)
Cash Collateral Offset	0	0
Net Amount Presented	(478)	(18)
Non-trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(791)	(120)
Gross Amounts Offset	313	94
Net Liabilities	(478)	(26)
Cash Collateral Offset	0	0
Net Amount Presented	(478)	(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

Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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