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Share-Based Compensation - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 17,051us-gaap_AllocatedShareBasedCompensationExpense $ 15,425us-gaap_AllocatedShareBasedCompensationExpense $ 13,399us-gaap_AllocatedShareBasedCompensationExpense
Share-based compensation capitalized into oil and gas properties 5,797us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount 5,078us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount 4,288us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
Stock option granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
Exercise price of stock options outstanding, minimum $ 6.97us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit    
Exercise price of stock options outstanding, maximum $ 53.20us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit    
Restricted stocks issued, shares 674,904us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod 848,498us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod 670,818us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Restricted stocks issued, value 24,593us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross 17,487us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross 21,085us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
Unrecognized compensation cost, weighted-average period of recognition 1 year 9 months 18 days    
Unrecognized compensation cost 22,354us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Adjustments to additional paid-in capital related to net tax effect of stock options exercises and restricted stock vesting (54)us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation (884)us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation 814us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
Adjustments to additional paid in capital, tax deficit of stock option exercises and restricted stock vesting 609us-gaap_AdjustmentsToAdditionalPaidInCapitalIncomeTaxDeficiencyFromShareBasedCompensation    
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation   15,405us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
13,308us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Restricted Stock [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period of stock options 1 year    
Restricted Stock [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period of stock options 3 years    
Stock Options[Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation   $ 20us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 91us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
2009 plan [Member] | All Employee [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period of stock options 5 years    
Period after which stock option will expire subsequent to award 10 years    
2009 plan [Member] | Non - Employee Directors [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period of stock options 3 years    
Period after which stock option will expire subsequent to award 10 years