Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Details)

v2.4.1.9
Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets $ 78,391us-gaap_DerivativeAssetsCurrent $ 324us-gaap_DerivativeAssetsCurrent  
Asset retirement obligation capitalized 9,295us-gaap_AssetRetirementObligationLiabilitiesIncurred 3,556us-gaap_AssetRetirementObligationLiabilitiesIncurred 2,195us-gaap_AssetRetirementObligationLiabilitiesIncurred
Level 1 [Member] | Diamondback Energy LLC [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity investment in Diamondback   178,708us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gpor_DiamondbackEnergyLlcMember
 
Level 1 [Member] | Fixed Price Swaps [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets 0us-gaap_DerivativeAssetsCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_DerivativeAssetsCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
 
Liabilities   0us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
 
Level 2 [Member] | Diamondback Energy LLC [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity investment in Diamondback   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gpor_DiamondbackEnergyLlcMember
 
Level 2 [Member] | Fixed Price Swaps [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets 102,839us-gaap_DerivativeAssetsCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
845us-gaap_DerivativeAssetsCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
 
Liabilities   23,646us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
 
Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Asset retirement obligation capitalized 9,300us-gaap_AssetRetirementObligationLiabilitiesIncurred
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
   
Level 3 [Member] | Diamondback Energy LLC [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity investment in Diamondback   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gpor_DiamondbackEnergyLlcMember
 
Level 3 [Member] | Fixed Price Swaps [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets 0us-gaap_DerivativeAssetsCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0us-gaap_DerivativeAssetsCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
 
Liabilities   $ 0us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= gpor_FixedPriceSwapsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member