Accounting for Derivative Instruments - Offsetting Assets (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commodity Contract
Offsetting Assets [Line Items]
Gross Assets	$ 4,189	$ 15,627
Gross Amounts Offset	(3,973)	(7,550)
Net Assets	216	8,077
Cash Collateral Offset	0	0
Net Amount Presented	216	8,077
Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	3,876	15,513
Gross Amounts Offset	(3,660)	(7,442)
Net Assets	216	8,071
Cash Collateral Offset	0	0
Net Amount Presented	216	8,071
Non-trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	3,642	11,564
Gross Amounts Offset	(3,562)	(6,898)
Net Assets	80	4,666
Cash Collateral Offset	0	0
Net Amount Presented	80	4,666
Trading Commodity Contract, Current
Offsetting Assets [Line Items]
Gross Assets	234	3,949
Gross Amounts Offset	(98)	(544)
Net Assets	136	3,405
Cash Collateral Offset	0	0
Net Amount Presented	136	3,405
Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	313	114
Gross Amounts Offset	(313)	(108)
Net Assets	0	6
Cash Collateral Offset	0	0
Net Amount Presented	0	6
Non-trading Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets	313	100
Gross Amounts Offset	(313)	(94)
Net Assets	0	6
Cash Collateral Offset	0	0
Net Amount Presented	0	6
Trading Commodity Contract, Noncurrent
Offsetting Assets [Line Items]
Gross Assets		14
Gross Amounts Offset		(14)
Net Assets		0
Cash Collateral Offset		0
Net Amount Presented		$ 0

Fair value of liability associated with financial asset or other 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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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Fair value, before effects of master netting arrangements, of a financial asset or other 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 assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative liability, of financial asset or other 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after effects of master netting arrangements, of financial asset or other 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 offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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