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Loans - Credit Quality (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile $ 1,785,001gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans $ 1,351,917gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
Purchased Credit Impaired 14,154us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet 7,498us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
Total loans held for investment 1,799,155us-gaap_LoansAndLeasesReceivableGrossCarryingAmount 1,359,415us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Grade 1 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 3,692gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 3,497gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember
Grade 2 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 5,338gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember		 7,800gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember
Grade 3 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 311,792gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 312,986gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember
Grade 4 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 1,321,507gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 966,207gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember
Grade 5 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 69,675gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 33,073gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember
Grade 6 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 46,243gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 12,466gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember
Grade 7 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 21,910gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 4,528gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember
Grade 8 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 4,844gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 8,843gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember
Grade 9 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile   2,517gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeNineMember
Commercial & Industrial [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 785,978gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 680,395gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Purchased Credit Impaired 2,432us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 895us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Total loans held for investment 788,410us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 681,290us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Commercial & Industrial [Member] | Grade 1 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 2,410gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 2,657gnbc_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 5,338gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember		 7,750gnbc_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 183,109gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 161,581gnbc_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 484,214gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 474,831gnbc_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 62,783gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 10,970gnbc_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 42,995gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 11,790gnbc_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 3,341gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 3,471gnbc_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 1,788gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 6,106gnbc_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   1,239gnbc_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 162,344gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 155,306gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
Purchased Credit Impaired 1,248us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 1,655us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
Total loans held for investment 163,592us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 156,961us-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 26,830gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 26,548gnbc_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 133,051gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 118,070gnbc_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 1,016gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 10,631gnbc_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		 57gnbc_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 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 331,745gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 262,335gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Purchased Credit Impaired 7,261us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 4,676us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Total loans held for investment 339,006us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 267,011us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Commerical real estate [Member] | Grade 3 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 33,347gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 37,750gnbc_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 283,401gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 213,946gnbc_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 1,935gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 8,752gnbc_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 2,680gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 569gnbc_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 9,812gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 502gnbc_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 570gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 816gnbc_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 240,594gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 139,971gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Purchased Credit Impaired 72us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 96us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Total loans held for investment 240,666us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 140,067us-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 7,605gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 12,058gnbc_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 222,209gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 124,972gnbc_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 3,692gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 2,323gnbc_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 6,148gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 31gnbc_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 940gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 587gnbc_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 253,925gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 106,186gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Purchased Credit Impaired 3,141us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 176us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Total loans held for investment 257,066us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 106,362us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Residential mortgage [Member] | Grade 1 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 285gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 292gnbc_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 57,945gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 71,264gnbc_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 192,565gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 32,735gnbc_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 99gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 149gnbc_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 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 1,307gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 418gnbc_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 1,277gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember		 1,328gnbc_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 10,415gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 7,724gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Total loans held for investment 10,415us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 7,724us-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 997gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember		 548gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeOneMember
Consumer and other [Member] | Grade 2 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile   50gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeTwoMember
Consumer and other [Member] | Grade 3 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile 2,956gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeThreeMember		 3,785gnbc_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 6,067gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFourMember		 1,653gnbc_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 150gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeFiveMember		 248gnbc_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 121gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSixMember		 107gnbc_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 29gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeSevenMember		 49gnbc_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		 6gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeEightMember
Consumer and other [Member] | Grade 9 [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Loans, credit risk profile   1,278gnbc_LoansAndLeasesReceivableGrossCarryingAmountExcludingPciLoans
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= gnbc_GradeNineMember
Originated [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 1,560,731us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember		 1,336,017us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
Originated [Member] | Commercial & Industrial [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 759,810us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 674,621us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Originated [Member] | Owner occupied commercial real estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 148,197us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 153,507us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
Originated [Member] | Commerical real estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 308,521us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 254,838us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Originated [Member] | Construction, land & land development [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 230,143us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 139,867us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Originated [Member] | Residential mortgage [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 107,275us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 105,888us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Originated [Member] | Consumer and other [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment 6,785us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 7,296us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_OriginatedLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Acquired [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Purchased Credit Impaired 14,154us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember		 7,498us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
Total loans held for investment 238,424us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember		 23,398us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
Acquired [Member] | Commercial & Industrial [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Purchased Credit Impaired 2,432us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 895us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Total loans held for investment 28,600us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 6,669us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Acquired [Member] | Owner occupied commercial real estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Purchased Credit Impaired 1,248us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 1,655us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
Total loans held for investment 15,395us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember		 3,454us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateOwnerOccupiedMember
Acquired [Member] | Commerical real estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Purchased Credit Impaired 7,261us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 4,676us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Total loans held for investment 30,485us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember		 12,173us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gnbc_CommercialRealEstateMultiFamilyMember
Acquired [Member] | Construction, land & land development [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Purchased Credit Impaired 72us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 96us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Total loans held for investment 10,523us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 200us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Acquired [Member] | Residential mortgage [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Purchased Credit Impaired 3,141us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 176us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Total loans held for investment 149,791us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 474us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Acquired [Member] | Consumer and other [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total loans held for investment $ 3,630us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 $ 428us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= gnbc_AcquiredLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember