Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 21,327	$ (10,981)	$ (88,498)
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	86,824us-gaap_CommodityContractAssetCurrent	92,771us-gaap_CommodityContractAssetCurrent	119,330us-gaap_CommodityContractAssetCurrent
Commodity derivative assets - noncurrent	243	507	1,365
Commodity derivative liabilities - current	(55,401)us-gaap_DerivativeLiabilitiesCurrent	(64,075)us-gaap_DerivativeLiabilitiesCurrent	(32,153)us-gaap_DerivativeLiabilitiesCurrent
Commodity derivative liabilities - noncurrent	(2,084)us-gaap_DerivativeLiabilitiesNoncurrent	(3,318)us-gaap_DerivativeLiabilitiesNoncurrent	(734)us-gaap_DerivativeLiabilitiesNoncurrent
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	61,969	56,633	92,915
Commodity derivative liabilities	(71,908)	(110,394)	(147,898)
Cash collateral	39,521	79,646	142,791
Total	29,582	25,885	87,808
Commodity \| Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	60,071	49,847	86,512
Commodity derivative liabilities	(12,768)	(36,722)	(109,973)
Cash collateral	39,521	79,646	142,791
Commodity derivative assets - current	86,824	92,771	119,330
Commodity \| Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	254	545	1,543
Commodity derivative liabilities	(11)	(38)	(178)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	243	507	1,365
Commodity \| Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,615	6,123	4,552
Commodity derivative liabilities	(57,016)	(70,198)	(36,705)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(55,401)	(64,075)	(32,153)
Commodity \| Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	29	118	308
Commodity derivative liabilities	(2,113)	(3,436)	(1,042)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (2,084)	$ (3,318)	$ (734)

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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