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Loans - Credit Quality (Details 9) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile $ 1,797,433gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans $ 1,785,001gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans  
Purchased Credit Impaired 13,409us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet 14,154us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet  
Total loans held for investment 1,810,842us-gaap_LoansAndLeasesReceivableGrossCarryingAmount 1,799,155us-gaap_LoansAndLeasesReceivableGrossCarryingAmount 1,404,275us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Grade 1 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 3,360gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 3,692gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		  
Grade 2 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 5,337gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember		 5,338gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember		  
Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 281,235gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 311,792gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,374,647gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 1,321,507gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 61,099gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 69,675gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Grade 6 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 46,238gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 46,243gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		  
Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 18,615gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 21,910gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 6,592gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 4,844gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		  
Grade 9 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 310gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeNineMember		    
Commercial & Industrial [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 742,325gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 785,978gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		  
Purchased Credit Impaired 2,055us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 2,432us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		  
Total loans held for investment 744,380us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 788,410us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 675,812us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Commercial & Industrial [Member] | Grade 1 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,868gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 2,410gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		  
Commercial & Industrial [Member] | Grade 2 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 5,337gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember		 5,338gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember		  
Commercial & Industrial [Member] | Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 153,619gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 183,109gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Commercial & Industrial [Member] | Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 483,546gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 484,214gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Commercial & Industrial [Member] | Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 51,412gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 62,783gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Commercial & Industrial [Member] | Grade 6 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 40,948gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 42,995gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		  
Commercial & Industrial [Member] | Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 2,276gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 3,341gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Commercial & Industrial [Member] | Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 3,009gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 1,788gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		  
Commercial & Industrial [Member] | Grade 9 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 310gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeNineMember		    
Owner occupied commercial real estate [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 165,541gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 162,344gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		  
Purchased Credit Impaired 1,063us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 1,248us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		  
Total loans held for investment 166,604us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 163,592us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 152,515us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
Owner occupied commercial real estate [Member] | Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 25,123gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 26,830gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Owner occupied commercial real estate [Member] | Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 139,236gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 133,051gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Owner occupied commercial real estate [Member] | Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 154gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 1,016gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Owner occupied commercial real estate [Member] | Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile   1,273gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Owner occupied commercial real estate [Member] | Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,028gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 174gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		  
Commerical real estate [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 359,926gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 331,745gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		  
Purchased Credit Impaired 7,145us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 7,261us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		  
Total loans held for investment 367,071us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 339,006us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 310,379us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Commerical real estate [Member] | Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 35,692gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 33,347gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Commerical real estate [Member] | Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 302,004gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 283,401gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Commerical real estate [Member] | Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 8,141gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 1,935gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Commerical real estate [Member] | Grade 6 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 4,652gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 2,680gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		  
Commerical real estate [Member] | Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 8,890gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 9,812gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Commerical real estate [Member] | Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 547gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 570gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		  
Construction, land & land development [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 273,068gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 240,594gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		  
Purchased Credit Impaired 57us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 72us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		  
Total loans held for investment 273,125us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 240,666us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 152,327us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Construction, land & land development [Member] | Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 7,203gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 7,605gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Construction, land & land development [Member] | Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 258,075gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 222,209gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Construction, land & land development [Member] | Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,153gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 3,692gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Construction, land & land development [Member] | Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 6,121gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 6,148gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Construction, land & land development [Member] | Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 516gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 940gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		  
Residential mortgage [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 246,502gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 253,925gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		  
Purchased Credit Impaired 3,089us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 3,141us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		  
Total loans held for investment 249,591us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 257,066us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 107,408us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Residential mortgage [Member] | Grade 1 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 283gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 285gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		  
Residential mortgage [Member] | Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 56,707gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 57,945gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Residential mortgage [Member] | Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 186,104gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 192,565gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Residential mortgage [Member] | Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 98gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 99gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Residential mortgage [Member] | Grade 6 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 518gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 447gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		  
Residential mortgage [Member] | Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,300gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 1,307gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Residential mortgage [Member] | Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,492gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 1,277gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		  
Consumer and other [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 10,071gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 10,415gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		  
Total loans held for investment 10,071us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 10,415us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 5,834us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Consumer and other [Member] | Grade 1 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 1,209gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 997gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		  
Consumer and other [Member] | Grade 3 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 2,891gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 2,956gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		  
Consumer and other [Member] | Grade 4 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 5,682gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 6,067gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		  
Consumer and other [Member] | Grade 5 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 141gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 150gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		  
Consumer and other [Member] | Grade 6 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 120gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 121gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		  
Consumer and other [Member] | Grade 7 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile 28gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 29gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		  
Consumer and other [Member] | Grade 8 [Member]      
Financing Receivable, Recorded Investment [Line Items]      
Loans, credit risk profile   $ 95gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember