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Income Taxes Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Income Tax Disclosure [Abstract] |  |  |
Net operating loss carryforward | $ 1,091 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards | $ 1,462 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards | $ 1,513 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
FASB ASC 718 compensation expense | 1,562 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost | 634 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost | 762 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost
AMT credit | 24,053 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | 7,968 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | 1,643 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax
Charitable contributions carryover | 150 us-gaap_DeferredTaxAssetsCharitableContributionCarryforwards | 25 us-gaap_DeferredTaxAssetsCharitableContributionCarryforwards | 5 us-gaap_DeferredTaxAssetsCharitableContributionCarryforwards
Unrealized loss on hedging activities | 0 us-gaap_DeferredTaxAssetsHedgingTransactions | 8,540 us-gaap_DeferredTaxAssetsHedgingTransactions | 3,836 us-gaap_DeferredTaxAssetsHedgingTransactions
Foreign tax credit carryforwards | 2,074 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign | 2,074 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign | 2,074 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign
Accrued liabilities | 1,260 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities | 0 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities | 0 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities
State net operating loss carryover | 2,627 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal | 4,408 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal | 4,315 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal
Total deferred tax assets | 32,817 us-gaap_DeferredTaxAssetsGross | 25,111 us-gaap_DeferredTaxAssetsGross | 14,148 us-gaap_DeferredTaxAssetsGross
Valuation allowance for deferred tax assets | (3,145)us-gaap_DeferredTaxAssetsValuationAllowance | (4,743)us-gaap_DeferredTaxAssetsValuationAllowance | (4,629)us-gaap_DeferredTaxAssetsValuationAllowance
Deferred tax assets, net of valuation allowance | 29,672 us-gaap_DeferredTaxAssetsNet | 20,368 us-gaap_DeferredTaxAssetsNet | 9,519 us-gaap_DeferredTaxAssetsNet
Oil and gas property basis difference | 183,767 gpor_DeferredTaxLiabilityOilAndGasPropertyBasisDifference | 72,173 gpor_DeferredTaxLiabilityOilAndGasPropertyBasisDifference | 15,049 gpor_DeferredTaxLiabilityOilAndGasPropertyBasisDifference
Investment in pass through entities | 38,315 us-gaap_DeferredTaxLiabilitiesInvestments | 8,799 us-gaap_DeferredTaxLiabilitiesInvestments | 3,618 us-gaap_DeferredTaxLiabilitiesInvestments
Non-oil and gas property basis difference | 849 gpor_DeferredTaxLiabilitiesNonoilAndGasPropertyBasisDifference | 249 gpor_DeferredTaxLiabilitiesNonoilAndGasPropertyBasisDifference | 227 gpor_DeferredTaxLiabilitiesNonoilAndGasPropertyBasisDifference
Investment in nonconsolidated affiliates | 0 us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates | 46,495 us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates | 9,232 us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates
Unrealized gain on hedging activities | 37,006 us-gaap_DeferredTaxLiabilitiesDerivatives | 0 us-gaap_DeferredTaxLiabilitiesDerivatives | 0 us-gaap_DeferredTaxLiabilitiesDerivatives
Total deferred tax liabilities | 259,937 us-gaap_DeferredTaxLiabilities | 127,716 us-gaap_DeferredTaxLiabilities | 28,126 us-gaap_DeferredTaxLiabilities
Net deferred tax liability | $ (230,265)us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent | $ (107,348)us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent | $ (18,607)us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign tax credit carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from share-based compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from accrued liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 + Details
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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
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- Definition Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=21917399&loc=d3e31931-109318 + Details
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