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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Assets: |  |
Total assets | $ 254,062 us-gaap_AssetsFairValueDisclosure | $ 316,249 us-gaap_AssetsFairValueDisclosure
Money market funds [Member] |  |
Assets: |  |
Total assets | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 253,320 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 315,507 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 1 [Member] |  |
Assets: |  |
Total assets | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 1 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 2 [Member] |  |
Assets: |  |
Total assets | 253,320 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 315,507 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 2 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 253,320 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 315,507 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 3 [Member] |  |
Assets: |  |
Total assets | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 3 [Member] | Money market funds [Member] |  |
Assets: |  |
Total assets | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Level 3 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | $ 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | $ 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration