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Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Assets: |  |
Total assets | $ 316,249 us-gaap_AssetsFairValueDisclosure | $ 4,339 us-gaap_AssetsFairValueDisclosure
Liabilities: |  |
Total liabilities |  | 8,327 us-gaap_LiabilitiesFairValueDisclosure
Money market funds [Member] |  |
Assets: |  |
Total assets | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember
Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 315,507 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember | 3,597 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember
Liabilities: |  |
Total liabilities |  | 8,327 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember
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 | 742 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_MoneyMarketFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 2 [Member] |  |
Assets: |  |
Total assets |  | 3,597 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Liabilities: |  |
Total liabilities |  | 8,327 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 [Member] | Commodity derivative instruments [Member] |  |
Assets: |  |
Total assets | 315,507 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 3,597 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Liabilities: |  |
Total liabilities |  | $ 8,327 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member

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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
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- Definition Fair value of financial and nonfinancial obligations. + 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
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