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Securities - Securities in Continuous Unrealized Loss Position (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
security
Dec. 31, 2013
security
Available-for-sale, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months $ 60,966gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost $ 95,031gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
Gross Unrealised Losses, Less than 12 Months (138)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss (1,244)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Fair Value, Less than 12 Months 60,828us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 93,787us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Amortized Cost, More than 12 Months 16,999gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost 4,481gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
Gross Unrealised Losses, More than 12 Months (345)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss (314)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Fair Value, More than 12 Months 16,654us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 4,167us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Held-to-maturity, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months 12,414gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost 29,093gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
Gross Unrealised Losses, Less than 12 Months (92)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss (1,135)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Fair Value, Less than 12 Months 12,322us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 27,958us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Amortized Cost, More than 12 Months 18,552gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost 1,022gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
Gross Unrealised Losses, More than 12 Months (468)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss (99)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Fair Value, More than 12 Months 18,084us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 923us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Investment Securities Continuous Unrealized Loss Position Qualitative Disclosure Abstract    
Securities in Unrealized Loss Position, Continuour Unrealized Loss as percent of Amortized Cost 10gnbc_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureContinuousUnrealizedLossAsPercentOfAmortizedCost 10gnbc_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureContinuousUnrealizedLossAsPercentOfAmortizedCost
Average loss on securities, percent 0.96%gnbc_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureAveragePercentageOfLoss 1.92%gnbc_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureAveragePercentageOfLoss
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions 12gnbc_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear 2gnbc_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
US Treasury and Government [Member]    
Available-for-sale, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months 37,049gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 25,008gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross Unrealised Losses, Less than 12 Months (85)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 (95)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fair Value, Less than 12 Months 36,964us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 24,913us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Available-for-sale, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months 20,403gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 51,576gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealised Losses, Less than 12 Months (53)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 (934)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value, Less than 12 Months 20,350us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 50,642us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Amortized Cost, More than 12 Months 4,440gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Gross Unrealised Losses, More than 12 Months (56)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Fair Value, More than 12 Months 4,384us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Held-to-maturity, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months   6,033gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealised Losses, Less than 12 Months   (429)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value, Less than 12 Months   5,604us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Amortized Cost, More than 12 Months 4,564gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 1,022gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealised Losses, More than 12 Months (122)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 (99)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value, More than 12 Months 4,442us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 923us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Collateralized Mortgage Backed Securities [Member]    
Available-for-sale, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months 3,514gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 18,447gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross Unrealised Losses, Less than 12 Months   (215)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Fair Value, Less than 12 Months 3,514us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 18,232us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Amortized Cost, More than 12 Months 12,559gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 4,481gnbc_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross Unrealised Losses, More than 12 Months (289)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 (314)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Fair Value, More than 12 Months 12,270us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 4,167us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Held-to-maturity, Continuous Unrealized Loss Position [Abstract]    
Amortized Cost, Less than 12 Months 12,414gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 23,060gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross Unrealised Losses, Less than 12 Months (92)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 (706)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Fair Value, Less than 12 Months 12,322us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 22,354us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Amortized Cost, More than 12 Months 13,988gnbc_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionMoreThan12MonthsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		  
Gross Unrealised Losses, More than 12 Months (346)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		  
Fair Value, More than 12 Months $ 13,642us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember