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Securities - Additional Information (Details 4) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Securities Pledged    
Securities pledged to various federal reserve districts, amortized cost $ 56,500,000gnbc_AmortizedCostOfSecuritiesPledgedToVariousFederalReserveDistricts $ 55,000,000gnbc_AmortizedCostOfSecuritiesPledgedToVariousFederalReserveDistricts
Securities pledged to various federal reserve districts, fair value 56,600,000gnbc_FairValueOfSecuritiesPledgedToVariousFederalReserveDistricts 54,800,000gnbc_FairValueOfSecuritiesPledgedToVariousFederalReserveDistricts
Repurchase Agreements [Member]    
Securities Pledged    
Amortized cost of securities pledged 17,100,000gnbc_SecurityOwnedAndPledgedAsCollateralAmortizedCost
/ us-gaap_FinancialInstrumentAxis
= us-gaap_RepurchaseAgreementsMember		 17,700,000gnbc_SecurityOwnedAndPledgedAsCollateralAmortizedCost
/ us-gaap_FinancialInstrumentAxis
= us-gaap_RepurchaseAgreementsMember
Securities pledged and available to be sold under repurchase agreements, fair value 17,300,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_FinancialInstrumentAxis
= us-gaap_RepurchaseAgreementsMember		 17,900,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_FinancialInstrumentAxis
= us-gaap_RepurchaseAgreementsMember
Derivative [Member]    
Securities Pledged    
Amortized cost of securities pledged 616,000gnbc_SecurityOwnedAndPledgedAsCollateralAmortizedCost
/ us-gaap_FinancialInstrumentAxis
= us-gaap_DerivativeMember		 669,000gnbc_SecurityOwnedAndPledgedAsCollateralAmortizedCost
/ us-gaap_FinancialInstrumentAxis
= us-gaap_DerivativeMember
Securities pledged and available to be sold under repurchase agreements, fair value $ 648,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_FinancialInstrumentAxis
= us-gaap_DerivativeMember		 $ 701,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_FinancialInstrumentAxis
= us-gaap_DerivativeMember