Derivatives (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	$ (10,981)	$ 31,055
Commodity derivative liabilities	0	0
Commodity derivative assets - current	92,771us-gaap_CommodityContractAssetCurrent	71,319us-gaap_CommodityContractAssetCurrent
Commodity derivative assets - noncurrent	507	246
Commodity derivative liabilities - current	(64,075)us-gaap_DerivativeLiabilitiesCurrent	(63,954)us-gaap_DerivativeLiabilitiesCurrent
Commodity derivative liabilities - noncurrent	(3,318)us-gaap_DerivativeLiabilitiesNoncurrent	(6,644)us-gaap_DerivativeLiabilitiesNoncurrent
Commodity Contract
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	56,633	70,870
Commodity derivative liabilities	(110,394)	(85,383)
Cash collateral	79,646	15,480
Total	25,885	967
Commodity Contract \| Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	49,847	69,289
Commodity derivative liabilities	(36,722)	(13,450)
Cash collateral	79,646	15,480
Commodity derivative assets - current	92,771	71,319
Commodity Contract \| Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	545	246
Commodity derivative liabilities	(38)	0
Cash collateral	0	0
Commodity derivative assets - noncurrent	507	246
Commodity Contract \| Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	6,123	1,286
Commodity derivative liabilities	(70,198)	(65,240)
Cash collateral	0	0
Commodity derivative liabilities - current	(64,075)	(63,954)
Commodity Contract \| Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	118	49
Commodity derivative liabilities	(3,436)	(6,693)
Cash collateral	0	0
Commodity derivative liabilities - noncurrent	$ (3,318)	$ (6,644)

Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

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

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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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, 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, after the 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, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the 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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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