Annual report pursuant to Section 13 and 15(d)

Long-Term Debt (Narrative) (Details)

v2.4.1.9
Long-Term Debt (Narrative) (Details) (USD $)
12 Months Ended 9 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Sep. 30, 2013
Dec. 27, 2013
Aug. 18, 2014
Dec. 21, 2012
Oct. 17, 2012
Oct. 09, 2012
Mar. 31, 2011
Apr. 23, 2014
Apr. 22, 2014
May 03, 2011
Sep. 30, 2010
Nov. 25, 2014
Feb. 26, 2015
Debt Instrument [Line Items]                                
Interest capitalized $ 9,687,000us-gaap_InterestCostsCapitalized $ 7,132,000us-gaap_InterestCostsCapitalized $ 0us-gaap_InterestCostsCapitalized                          
Interest expense 23,986,000us-gaap_InterestExpense 17,490,000us-gaap_InterestExpense 7,458,000us-gaap_InterestExpense                          
Debt covenant ratio for EBITDAX       3.00gpor_DebtInstrumentCovenantComplianceEarningsBeforeIncomeTaxDepreciationDepletionAmortizationAndExplorationExpensesRatio                        
Long-tern debt 701,826,000us-gaap_LongTermDebt                              
Unamortized discount 2,800,000us-gaap_DebtInstrumentUnamortizedDiscount                              
Senior Notes [Member]                                
Debt Instrument [Line Items]                                
Stated interest rate       7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
  7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
                   
Long-tern debt 600,000,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
[1] 300,000,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
[1]       600,000,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
[1]                    
Debt issued           300,000,000.0us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
50,000,000.0us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
250,000,000.0us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
               
Redemption of principal amount plus aggregate net proceeds                 100.00%gpor_DebtInstrumentRedemptionPercentageOfPrincipalAmountPlusAggregateNetProceeds
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
             
Discount issue price, price           106.00%gpor_DebtInstrumentDiscountIssuePricePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
  98.534%gpor_DebtInstrumentDiscountIssuePricePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
               
Premium issue price, percent             101.00%gpor_DebtInstrumentPremiumIssuePricePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
                 
Unamortized discount               3,700,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
               
Effective interest rate           6.561%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
7.531%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
8.00%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
               
Unamortized premium           18,000,000us-gaap_DebtInstrumentUnamortizedPremium
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
500,000us-gaap_DebtInstrumentUnamortizedPremium
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
                 
Building Loans [Member]                                
Debt Instrument [Line Items]                                
Stated interest rate                   5.82%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= gpor_BuildingLoansMember
           
Long-tern debt 1,826,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gpor_BuildingLoansMember
[2] 1,995,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gpor_BuildingLoansMember
[2]               2,400,000us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gpor_BuildingLoansMember
           
Loan, periodic payment       22,000us-gaap_DebtInstrumentPeriodicPayment
/ us-gaap_LongtermDebtTypeAxis
= gpor_BuildingLoansMember
                       
Minimum [Member] | Senior Notes [Member]                                
Debt Instrument [Line Items]                                
Percentage of notes required to be outstanding for redemption                 65.00%gpor_DebtInstrumentOutstandingNotesRequiredForRedemption
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
             
Maximum [Member] | Senior Notes [Member]                                
Debt Instrument [Line Items]                                
Redemption of principal amount plus aggregate net proceeds                 35.00%gpor_DebtInstrumentRedemptionPercentageOfPrincipalAmountPlusAggregateNetProceeds
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
             
Nova Scotia, Amegy, KeyBank [Member]                                
Debt Instrument [Line Items]                                
Revolving credit facility         1,500,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
              350,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
     
Borrowing capacity         150,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
          275,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
150,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
  50,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
   
Debt instrument, description of rate LIBOR01                              
Nova Scotia, Amegy, KeyBank [Member] | Letter of Credit [Member]                                
Debt Instrument [Line Items]                                
Borrowing capacity                     70,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
20,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
       
Debt covenant ratio for EBITDAX                             2.00gpor_DebtInstrumentCovenantComplianceEarningsBeforeIncomeTaxDepreciationDepletionAmortizationAndExplorationExpensesRatio
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
 
Bank of Nova Scotia [Member]                                
Debt Instrument [Line Items]                                
Borrowing capacity 450,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_BankofNovaScotiaMember
                             
Base Rate Loans [Member] | Nova Scotia, Amegy, KeyBank [Member]                                
Debt Instrument [Line Items]                                
Applicable rate, minimum         0.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_DebtInstrumentAxis
= gpor_BaseRateLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                     
Applicable rate, maximum         1.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_DebtInstrumentAxis
= gpor_BaseRateLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                     
Second Amendment of Restated Credit Agreement [Member] | Nova Scotia, Amegy, KeyBank [Member]                                
Debt Instrument [Line Items]                                
Borrowing capacity 450,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= gpor_SecondAmendmentofRestatedCreditAgreementMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                             
Future borrowings available 306,400,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= gpor_SecondAmendmentofRestatedCreditAgreementMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                             
Second Amendment of Restated Credit Agreement [Member] | Nova Scotia, Amegy, KeyBank [Member] | Letter of Credit [Member]                                
Debt Instrument [Line Items]                                
Borrowing capacity                     125,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_DebtInstrumentAxis
= gpor_SecondAmendmentofRestatedCreditAgreementMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
         
Debt covenant ratio for EBITDAX                             3.50gpor_DebtInstrumentCovenantComplianceEarningsBeforeIncomeTaxDepreciationDepletionAmortizationAndExplorationExpensesRatio
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_DebtInstrumentAxis
= gpor_SecondAmendmentofRestatedCreditAgreementMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
 
Debt covenant ratio for future EBITDAX                             3.25gpor_DebtInstrumentCovenantComplianceFutureEarningsBeforeIncomeTaxDepreciationDepletionAmortizationandExplorationExpensesRatio
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_DebtInstrumentAxis
= gpor_SecondAmendmentofRestatedCreditAgreementMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
 
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
                             
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
                             
Euro Dollar Loans [Member] | Nova Scotia, Amegy, KeyBank [Member]                                
Debt Instrument [Line Items]                                
Applicable rate, minimum         1.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_DebtInstrumentAxis
= gpor_EuroDollarLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                     
Applicable rate, maximum         2.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_DebtInstrumentAxis
= gpor_EuroDollarLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                     
Effective interest rate 1.91%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= gpor_EuroDollarLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
                             
Federal Funds Rate [Member] | Base Rate Loans [Member] | Nova Scotia, Amegy, KeyBank [Member]                                
Debt Instrument [Line Items]                                
Basis spread         0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= gpor_BaseRateLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
/ us-gaap_VariableRateAxis
= gpor_FederalFundsRateMember
                     
Eurodollar [Member] | Base Rate Loans [Member] | Nova Scotia, Amegy, KeyBank [Member]                                
Debt Instrument [Line Items]                                
Basis spread         1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= gpor_BaseRateLoansMember
/ us-gaap_LineOfCreditFacilityAxis
= gpor_NovaScotiaAmegyKeybankMember
/ us-gaap_VariableRateAxis
= us-gaap_EurodollarMember
                     
Subsequent Event [Member] | Bank of Nova Scotia [Member]                                
Debt Instrument [Line Items]                                
Borrowing capacity                               $ 575,000,000.0us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpor_BankofNovaScotiaMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
[1] On October 17, 2012, the Company issued $250.0 million in aggregate principal amount of October Notes to qualified institutional buyers pursuant to Rule 144A under the Securities Act and to certain non-U.S. persons in accordance with Regulation S under the Securities Act, (the "October Notes Offering") under an indenture among the Company, its subsidiary guarantors and Wells Fargo Bank, National Association, as the trustee, (the "senior note indenture"). On December 21, 2012, the Company issued an additional $50.0 million in aggregate principal amount of December Notes to qualified institutional buyers pursuant to Rule 144A under the Securities Act and to certain non-U.S. persons in accordance with Regulation S under the Securities Act ("the December Notes Offering"). The December Notes were issued as additional securities under the senior note indenture. The October Notes Offering and the December Notes Offering are collectively referred to as the "Notes Offerings". The Company used a portion of the net proceeds from the October Notes Offering to repay all amounts outstanding at such time under its revolving credit facility. The Company used the remaining net proceeds of October Notes Offering and the net proceeds of the December Notes Offering for general corporate purposes, which included funding a portion of its 2013 capital development plan. The October Notes and the December Notes were exchanged for substantially identical notes in the same aggregate principal amount that were registered under the Securities Act in October 2013 (the "Exchange Notes").On August 18, 2014, the Company issued an additional $300.0 million in aggregate principal amount of senior unsecured notes due 2020 (the "August Notes") to qualified institutional buyers pursuant to Rule 144A under the Securities Act and to certain non-U.S. persons in accordance with Regulation S under the Securities Act ("the August Notes Offering"). The August Notes were issued as additional securities under the senior note indenture. The Company used a portion of the net proceeds from the August Notes Offering to repay all amounts outstanding at such time under its revolving credit facility. The Company used the remaining net proceeds of the August Notes Offering for general corporate purposes, including funding a portion of its 2014 capital development plans. The October Notes Offering, December Notes Offering and the August Notes Offering are collectively referred to as the "Notes Offerings" and the Exchange Notes, and the August Notes are collectively referred to as the "Notes".In connection with the issuance of the August Notes, the Company and the subsidiary guarantors entered into a registration rights agreement with the initial purchasers on August 18, 2014, pursuant to which the Company and the subsidiary guarantors have agreed to file a registration statement with respect to an offer to exchange the August Notes for a new issue of substantially identical debt securities registered under the Securities Act. The registration statement relating to the exchange offer for the August Notes was filed on November 6, 2014, as amended on February 3, 2015, and declared effective by the SEC on February 4, 2015. The exchange offer for the August Note is expected to be completed on or about March 10, 2015.Under the senior note indenture, interest on the Notes accrues at a rate of 7.75% per annum on the outstanding principal amount from October 17, 2012, payable semi-annually on May 1 and November 1 of each year, commencing on May 1, 2013. The Notes are the Company's senior unsecured obligations and rank equally in the right of payment with all of the Company's other senior indebtedness and senior in right of payment to any future subordinated indebtedness. All of the Company's existing and future restricted subsidiaries that guarantee the Company's secured revolving credit facility or certain other debt guarantee the Notes; provided, however, that the Notes are not guaranteed by Grizzly Holdings, Inc. and will not be guaranteed by any of the Company's future unrestricted subsidiaries. The Company may redeem some or all of the Notes at any time on or after November 1, 2016, at the redemption prices listed in the senior note indenture. Prior to November 1, 2016, the Company may redeem the Notes at a price equal to 100% of the principal amount plus a “make-whole” premium. In addition, prior to November 1, 2015, the Company may redeem up to 35% of the aggregate principal amount of the Notes with the net proceeds of certain equity offerings, provided that at least 65% of the aggregate principal amount of the Notes initially issued remains outstanding immediately after such redemption.
[2] In March 2011, the Company entered into a new building loan agreement for the office building it occupies in Oklahoma City, Oklahoma. The new loan agreement refinanced the $2.4 million outstanding under the previous building loan agreement. The new agreement matures in February 2016 and bears interest at the rate of 5.82% per annum. The new building loan requires monthly interest and principal payments of approximately $22,000 and is collateralized by the Oklahoma City office building and associated land.