Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	$ 272,675us-gaap_DerivativeAssets	$ 322,337us-gaap_DerivativeAssets
Total liabilities	(13,843)us-gaap_DerivativeLiabilities	(988)us-gaap_DerivativeLiabilities
Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments	272,675	322,337
Total assets	272,675	322,337
Derivative instruments	(12,870)	(988)
Noncurrent derivative instruments	(973)
Total liabilities	(13,843)	(988)
Net derivative asset	258,832	321,349
Recurring \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments	256,419	294,865
Total assets	256,419	294,865
Derivative instruments	0	2,048
Noncurrent derivative instruments	0
Total liabilities	0	2,048
Net derivative asset	256,419	296,913
Recurring \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments	16,256	27,472
Total assets	16,256	27,472
Derivative instruments	(12,870)	(3,036)
Noncurrent derivative instruments	(973)
Total liabilities	(13,843)	(3,036)
Net derivative asset	$ 2,413	$ 24,436

Fair value, after the 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 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 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, 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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- Definition

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. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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- Definition

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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- Definition

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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