Long-Term Debt (Narrative) (Details) (USD $)
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12 Months Ended |
9 Months Ended |
0 Months Ended |
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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Sep. 30, 2013
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Dec. 27, 2013
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Aug. 18, 2014
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Dec. 21, 2012
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Oct. 17, 2012
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Oct. 09, 2012
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Mar. 31, 2011
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Apr. 23, 2014
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Apr. 22, 2014
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May 03, 2011
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Sep. 30, 2010
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Nov. 25, 2014
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Feb. 26, 2015
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Debt Instrument [Line Items] |
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Interest capitalized |
$ 9,687,000us-gaap_InterestCostsCapitalized
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$ 7,132,000us-gaap_InterestCostsCapitalized
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$ 0us-gaap_InterestCostsCapitalized
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Interest expense |
23,986,000us-gaap_InterestExpense
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17,490,000us-gaap_InterestExpense
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7,458,000us-gaap_InterestExpense
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Debt covenant ratio for EBITDAX |
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3.00gpor_DebtInstrumentCovenantComplianceEarningsBeforeIncomeTaxDepreciationDepletionAmortizationAndExplorationExpensesRatio
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Long-tern debt |
701,826,000us-gaap_LongTermDebt
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Unamortized discount |
2,800,000us-gaap_DebtInstrumentUnamortizedDiscount
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Senior Notes [Member] |
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Debt Instrument [Line Items] |
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Stated interest rate |
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7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Long-tern debt |
600,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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[1] |
300,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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[1] |
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600,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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[1] |
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Debt issued |
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300,000,000.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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50,000,000.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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250,000,000.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Redemption of principal amount plus aggregate net proceeds |
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100.00%gpor_DebtInstrumentRedemptionPercentageOfPrincipalAmountPlusAggregateNetProceeds / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Discount issue price, price |
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106.00%gpor_DebtInstrumentDiscountIssuePricePercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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98.534%gpor_DebtInstrumentDiscountIssuePricePercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Premium issue price, percent |
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101.00%gpor_DebtInstrumentPremiumIssuePricePercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Unamortized discount |
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3,700,000us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Effective interest rate |
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6.561%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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7.531%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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8.00%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Unamortized premium |
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18,000,000us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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500,000us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
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Building Loans [Member] |
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Debt Instrument [Line Items] |
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Stated interest rate |
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5.82%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = gpor_BuildingLoansMember
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Long-tern debt |
1,826,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = gpor_BuildingLoansMember
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[2] |
1,995,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = gpor_BuildingLoansMember
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[2] |
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2,400,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = gpor_BuildingLoansMember
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Loan, periodic payment |
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22,000us-gaap_DebtInstrumentPeriodicPayment / us-gaap_LongtermDebtTypeAxis = gpor_BuildingLoansMember
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Minimum [Member] | Senior Notes [Member] |
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Debt Instrument [Line Items] |
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Percentage of notes required to be outstanding for redemption |
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65.00%gpor_DebtInstrumentOutstandingNotesRequiredForRedemption / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
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Maximum [Member] | Senior Notes [Member] |
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Debt Instrument [Line Items] |
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Redemption of principal amount plus aggregate net proceeds |
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35.00%gpor_DebtInstrumentRedemptionPercentageOfPrincipalAmountPlusAggregateNetProceeds / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
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Nova Scotia, Amegy, KeyBank [Member] |
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Debt Instrument [Line Items] |
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Revolving credit facility |
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1,500,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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350,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Borrowing capacity |
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150,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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275,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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150,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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50,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Debt instrument, description of rate |
LIBOR01
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Nova Scotia, Amegy, KeyBank [Member] | Letter of Credit [Member] |
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Debt Instrument [Line Items] |
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Borrowing capacity |
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70,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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20,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Debt covenant ratio for EBITDAX |
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2.00gpor_DebtInstrumentCovenantComplianceEarningsBeforeIncomeTaxDepreciationDepletionAmortizationAndExplorationExpensesRatio / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Bank of Nova Scotia [Member] |
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Debt Instrument [Line Items] |
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Borrowing capacity |
450,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_BankofNovaScotiaMember
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Base Rate Loans [Member] | Nova Scotia, Amegy, KeyBank [Member] |
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Debt Instrument [Line Items] |
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Applicable rate, minimum |
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0.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = gpor_BaseRateLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Applicable rate, maximum |
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1.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = gpor_BaseRateLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Second Amendment of Restated Credit Agreement [Member] | Nova Scotia, Amegy, KeyBank [Member] |
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Debt Instrument [Line Items] |
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Borrowing capacity |
450,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Future borrowings available |
306,400,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Second Amendment of Restated Credit Agreement [Member] | Nova Scotia, Amegy, KeyBank [Member] | Letter of Credit [Member] |
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Debt Instrument [Line Items] |
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Borrowing capacity |
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125,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Debt covenant ratio for EBITDAX |
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3.50gpor_DebtInstrumentCovenantComplianceEarningsBeforeIncomeTaxDepreciationDepletionAmortizationAndExplorationExpensesRatio / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Debt covenant ratio for future EBITDAX |
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3.25gpor_DebtInstrumentCovenantComplianceFutureEarningsBeforeIncomeTaxDepreciationDepletionAmortizationandExplorationExpensesRatio / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Letters of Credit Outstanding, Amount |
43,600,000us-gaap_LettersOfCreditOutstandingAmount / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Disposition costs, maximum expenses allowed |
3,000,000gpor_DebtInstrumentDispositionCostsMaximumExpensesAllowed / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_DebtInstrumentAxis = gpor_SecondAmendmentofRestatedCreditAgreementMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Euro Dollar Loans [Member] | Nova Scotia, Amegy, KeyBank [Member] |
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Debt Instrument [Line Items] |
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Applicable rate, minimum |
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1.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = gpor_EuroDollarLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Applicable rate, maximum |
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2.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = gpor_EuroDollarLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Effective interest rate |
1.91%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = gpor_EuroDollarLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember
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Federal Funds Rate [Member] | Base Rate Loans [Member] | Nova Scotia, Amegy, KeyBank [Member] |
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Debt Instrument [Line Items] |
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Basis spread |
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0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = gpor_BaseRateLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember / us-gaap_VariableRateAxis = gpor_FederalFundsRateMember
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Eurodollar [Member] | Base Rate Loans [Member] | Nova Scotia, Amegy, KeyBank [Member] |
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Debt Instrument [Line Items] |
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Basis spread |
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1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = gpor_BaseRateLoansMember / us-gaap_LineOfCreditFacilityAxis = gpor_NovaScotiaAmegyKeybankMember / us-gaap_VariableRateAxis = us-gaap_EurodollarMember
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Subsequent Event [Member] | Bank of Nova Scotia [Member] |
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Debt Instrument [Line Items] |
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Borrowing capacity |
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$ 575,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = gpor_BankofNovaScotiaMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
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