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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, 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]      
Compensation costs expensed $ 7.2us-gaap_AllocatedShareBasedCompensationExpense $ 5.5us-gaap_AllocatedShareBasedCompensationExpense $ 5.5us-gaap_AllocatedShareBasedCompensationExpense
Unrecognized compensation costs $ 6.6us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Compensation expected to be recognized, weighted-average period in years 1 year 9 months 18 days    
Potential tax benefit, percentage 37.50%gpre_ShareBasedCompensationPotentialTaxBenefitRealizableForAnticipatedTaxDeductionsExerciseOfShareBasedPaymentArrangementsPercentage    
Equity Incentive Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation arrangement by share-based payment award, number of shares authorized 3,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= gpre_EquityIncentivePlansMember
   
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Exercisable options, expiration period, years 8 years    
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Exercisable options, expiration period, years 5 years