<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Version>2.4.0.8</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>234</ContextCount>
  <ElementCount>393</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>67</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document And Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - Consolidated Statements Of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedStatementsOfIncome</Role>
      <ShortName>Consolidated Statements Of Income</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00205 - Statement - Consolidated Statements Of Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedStatementsOfIncomeParenthetical</Role>
      <ShortName>Consolidated Statements Of Income (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00300 - Statement - Consolidated Statements Of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Consolidated Statements Of Comprehensive Income</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00400 - Statement - Consolidated Statements Of Changes In Stockholders' Equity And Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquityAndComprehensiveIncomeLoss</Role>
      <ShortName>Consolidated Statements Of Changes In Stockholders' Equity And Comprehensive Income (Loss)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>00405 - Statement - Consolidated Statements Of Changes In Stockholders' Equity And Comprehensive Income (Loss) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquityAndComprehensiveIncomeLossParenthetical</Role>
      <ShortName>Consolidated Statements Of Changes In Stockholders' Equity And Comprehensive Income (Loss) (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>00500 - Statement - Consolidated Statements Of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements Of Cash Flows</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Organization, Operations and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOrganizationOperationsAndBasisOfPresentation</Role>
      <ShortName>Organization, Operations and Basis of Presentation</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecurities</Role>
      <ShortName>Securities</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Loans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoans</Role>
      <ShortName>Loans</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherRealEstateOwned</Role>
      <ShortName>Other Real Estate Owned</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherIntangibleAssets</Role>
      <ShortName>Other Intangible Assets</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureBorrowings</Role>
      <ShortName>Borrowings</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Subordinated Debentures And Trust Preferred Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSubordinatedDebenturesAndTrustPreferredSecurities</Role>
      <ShortName>Subordinated Debentures And Trust Preferred Securities</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Commitments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureCommitments</Role>
      <ShortName>Commitments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstruments</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Derivatives And Hedging Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureDerivativesAndHedgingActivities</Role>
      <ShortName>Derivatives And Hedging Activities</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Capital Ratios</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureCapitalRatios</Role>
      <ShortName>Capital Ratios</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>20102 - Disclosure - Organization, Operations, And Basis Of Presenstation (Policy) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOrganizationOperationsAndBasisOfPresenstationPolicyDetails</Role>
      <ShortName>Organization, Operations, And Basis Of Presenstation (Policy) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>30103 - Disclosure - Organization, Operations And Basis Of Presentation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOrganizationOperationsAndBasisOfPresentationTables</Role>
      <ShortName>Organization, Operations And Basis Of Presentation (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>30203 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>30303 - Disclosure - Loans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansTables</Role>
      <ShortName>Loans (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherRealEstateOwnedTables</Role>
      <ShortName>Other Real Estate Owned (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherIntangibleAssetsTables</Role>
      <ShortName>Other Intangible Assets (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Subordinated Debentures And Trust Preferred Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSubordinatedDebenturesAndTrustPreferredSecuritiesTables</Role>
      <ShortName>Subordinated Debentures And Trust Preferred Securities (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Commitments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureCommitmentsTables</Role>
      <ShortName>Commitments (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>30903 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsTables</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Derivatives And Hedging Activities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureDerivativesAndHedgingActivitiesTables</Role>
      <ShortName>Derivatives And Hedging Activities (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>31103 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>31203 - Disclosure - Capital Ratios (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureCapitalRatiosTables</Role>
      <ShortName>Capital Ratios (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Organizations, Operations And Basis Of Presentation (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOrganizationsOperationsAndBasisOfPresentationNarrativeDetails</Role>
      <ShortName>Organizations, Operations And Basis Of Presentation (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>40102 - Disclosure - Organizations, Operations And Basis Of Presentation (Earnings Loss Per Common Share) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOrganizationsOperationsAndBasisOfPresentationEarningsLossPerCommonShareDetails</Role>
      <ShortName>Organizations, Operations And Basis Of Presentation (Earnings Loss Per Common Share) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Securities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesNarrativeDetails</Role>
      <ShortName>Securities (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40202 - Disclosure - Securities (Fair Value Of Available For Sale Debt Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesFairValueOfAvailableForSaleDebtSecuritiesDetails</Role>
      <ShortName>Securities (Fair Value Of Available For Sale Debt Securities) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40203 - Disclosure - Securities (Unrecognized Gains And Losses Securities Held To Maturity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesUnrecognizedGainsAndLossesSecuritiesHeldToMaturityDetails</Role>
      <ShortName>Securities (Unrecognized Gains And Losses Securities Held To Maturity) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40204 - Disclosure - Securities (Proceeds From Sales And Calls Of Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesProceedsFromSalesAndCallsOfSecuritiesDetails</Role>
      <ShortName>Securities (Proceeds From Sales And Calls Of Securities) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40205 - Disclosure - Securities (Investments Classified by Contractual Maturity Date) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesInvestmentsClassifiedByContractualMaturityDateDetails</Role>
      <ShortName>Securities (Investments Classified by Contractual Maturity Date) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40206 - Disclosure - Securities (Fair Value And The Unrealized Loss On Temporarily Impaired Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSecuritiesFairValueAndUnrealizedLossOnTemporarilyImpairedSecuritiesDetails</Role>
      <ShortName>Securities (Fair Value And The Unrealized Loss On Temporarily Impaired Securities) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Loans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansNarrativeDetails</Role>
      <ShortName>Loans (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40302 - Disclosure - Loans (Loans Held For Investment By Loan Type) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansLoansHeldForInvestmentByLoanTypeDetails</Role>
      <ShortName>Loans (Loans Held For Investment By Loan Type) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40303 - Disclosure - Loans (Allowance For Loan Losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansAllowanceForLoanLossesDetails</Role>
      <ShortName>Loans (Allowance For Loan Losses) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40304 - Disclosure - Loans (Allowance For Loan Losses And Loans Held For Investment By Portfolio Segment) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansAllowanceForLoanLossesAndLoansHeldForInvestmentByPortfolioSegmentDetails</Role>
      <ShortName>Loans (Allowance For Loan Losses And Loans Held For Investment By Portfolio Segment) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40305 - Disclosure - Loans (Schedule Of Impaired Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansScheduleOfImpairedLoansDetails</Role>
      <ShortName>Loans (Schedule Of Impaired Loans) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40306 - Disclosure - Loans (Summary Of Past Due Loans By Class) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansSummaryOfPastDueLoansByClassDetails</Role>
      <ShortName>Loans (Summary Of Past Due Loans By Class) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40307 - Disclosure - Loans (Summary Of The Risk Category Of Loans By Class Of Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansSummaryOfRiskCategoryOfLoansByClassOfLoansDetails</Role>
      <ShortName>Loans (Summary Of The Risk Category Of Loans By Class Of Loans) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>40308 - Disclosure - Loans (Loans By Class Modified As Troubled Debt Restructurings) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureLoansLoansByClassModifiedAsTroubledDebtRestructuringsDetails</Role>
      <ShortName>Loans (Loans By Class Modified As Troubled Debt Restructurings) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Other Real Estate Owned (Carrying Amount Of Other Real Estate Owned) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherRealEstateOwnedCarryingAmountOfOtherRealEstateOwnedDetails</Role>
      <ShortName>Other Real Estate Owned (Carrying Amount Of Other Real Estate Owned) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40402 - Disclosure - Other Real Estate Owned (Expenses Related To Foreclosed Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherRealEstateOwnedExpensesRelatedToForeclosedAssetsDetails</Role>
      <ShortName>Other Real Estate Owned (Expenses Related To Foreclosed Assets) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Other Intangible Assets (Schedule Of Finite Lived Intangible Assets And Related Accumulated Amortization) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherIntangibleAssetsScheduleOfFiniteLivedIntangibleAssetsAndRelatedAccumulatedAmortizationDetails</Role>
      <ShortName>Other Intangible Assets (Schedule Of Finite Lived Intangible Assets And Related Accumulated Amortization) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40502 - Disclosure - Other Intangible Assets (Amortization Of Intangible Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureOtherIntangibleAssetsAmortizationOfIntangibleAssetsDetails</Role>
      <ShortName>Other Intangible Assets (Amortization Of Intangible Assets) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40604 - Disclosure - Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureBorrowingsDetails</Role>
      <ShortName>Borrowings (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Subordinated Debentures And Trust Preferred Securities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSubordinatedDebenturesAndTrustPreferredSecuritiesNarrativeDetails</Role>
      <ShortName>Subordinated Debentures And Trust Preferred Securities (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40702 - Disclosure - Subordinated Debentures And Trust Preferred Securities (Schedule Of Subordinated Debenture Issuance) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureSubordinatedDebenturesAndTrustPreferredSecuritiesScheduleOfSubordinatedDebentureIssuanceDetails</Role>
      <ShortName>Subordinated Debentures And Trust Preferred Securities (Schedule Of Subordinated Debenture Issuance) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureCommitmentsDetails</Role>
      <ShortName>Commitments (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsNarrativeDetails</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>40902 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>40903 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Fair Value Measurements Using Significant Unobservable Inputs Level 3) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsFairValueMeasurementsUsingSignificantUnobservableInputsLevel3Details</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Fair Value Measurements Using Significant Unobservable Inputs Level 3) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>40904 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Quantitative Information About Level 3 Fair Value Measurements) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsQuantitativeInformationAboutLevel3FairValueMeasurementsDetails</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Quantitative Information About Level 3 Fair Value Measurements) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>40905 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Impaired Loans Measured At Fair Value On A Non-Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsImpairedLoansMeasuredAtFairValueOnNonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Impaired Loans Measured At Fair Value On A Non-Recurring Basis) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>40906 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Nonfinancial Assets And Liabilities Measured On A Nonrecurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsNonfinancialAssetsAndLiabilitiesMeasuredOnNonrecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Nonfinancial Assets And Liabilities Measured On A Nonrecurring Basis) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>40907 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Estimated Fair Values And Related Carrying Amounts Of Financial Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureFairValueMeasurementsAndFairValueOfFinancialInstrumentsEstimatedFairValuesAndRelatedCarryingAmountsOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Measurements And Fair Value Of Financial Instruments (Estimated Fair Values And Related Carrying Amounts Of Financial Instruments) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Derivatives And Hedging Activities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureDerivativesAndHedgingActivitiesNarrativeDetails</Role>
      <ShortName>Derivatives And Hedging Activities (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>41002 - Disclosure - Derivatives And Hedging Activities (Schedule Of Derivative Financial Instruments As Well As Their Classification On The Balance Sheet) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureDerivativesAndHedgingActivitiesScheduleOfDerivativeFinancialInstrumentsAsWellAsTheirClassificationOnBalanceSheetDetails</Role>
      <ShortName>Derivatives And Hedging Activities (Schedule Of Derivative Financial Instruments As Well As Their Classification On The Balance Sheet) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>41003 - Disclosure - Derivatives And Hedging Activities (Schedule Of Derivative Financial Instruments On Both Comprehensive Income And Net Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureDerivativesAndHedgingActivitiesScheduleOfDerivativeFinancialInstrumentsOnBothComprehensiveIncomeAndNetIncomeDetails</Role>
      <ShortName>Derivatives And Hedging Activities (Schedule Of Derivative Financial Instruments On Both Comprehensive Income And Net Income) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Stock-Based Compensation (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureStockBasedCompensationNarrativeDetails</Role>
      <ShortName>Stock-Based Compensation (Narrative) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>41102 - Disclosure - Stock-Based Compensation (Stock-Based Compensation Change During The Period) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureStockBasedCompensationStockBasedCompensationChangeDuringPeriodDetails</Role>
      <ShortName>Stock-Based Compensation (Stock-Based Compensation Change During The Period) (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>41201 - Disclosure - Capital Ratios (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gbnk.com/role/DisclosureCapitalRatiosDetails</Role>
      <ShortName>Capital Ratios (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">Element gbnk_ImpairedFinancingReveivableCarriedAtCost had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">Element gbnk_ImpairedFinancingReveivableCarriedAtFairValue had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">Element gbnk_IncomeTaxEffectRecognizedOnConsolidatedBalanceSheetForShareBasedCompensationArrangements had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">Element us-gaap_Assets had a mix of decimals attribute values: -9 -3.</Log>
    <Log type="Info">Element us-gaap_FederalHomeLoanBankAdvancesLongTerm had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">Element us-gaap_ImpairedFinancingReceivableRecordedInvestment had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">Element us-gaap_RealEstateAcquiredThroughForeclosure had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">Element us-gaap_SubordinatedDebt had a mix of decimals attribute values: -3 0.</Log>
    <Log type="Info">'Monetary' elements on report '00100 - Statement - Consolidated Balance Sheets' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '40101 - Disclosure - Organizations, Operations And Basis Of Presentation (Narrative) (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '40305 - Disclosure - Loans (Schedule Of Impaired Loans) (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '40904 - Disclosure - Fair Value Measurements And Fair Value Of Financial Instruments (Quantitative Information About Level 3 Fair Value Measurements) (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">Process Flow-Through: 00100 - Statement - Consolidated Balance Sheets</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Mar. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 31, 2012'</Log>
    <Log type="Info">Process Flow-Through: 00105 - Statement - Consolidated Balance Sheets (Parenthetical)</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Apr. 03, 2014'</Log>
    <Log type="Info">Process Flow-Through: 00200 - Statement - Consolidated Statements Of Income</Log>
    <Log type="Info">Process Flow-Through: 00205 - Statement - Consolidated Statements Of Income (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 00300 - Statement - Consolidated Statements Of Comprehensive Income</Log>
    <Log type="Info">Process Flow-Through: 00405 - Statement - Consolidated Statements Of Changes In Stockholders' Equity And Comprehensive Income (Loss) (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 00500 - Statement - Consolidated Statements Of Cash Flows</Log>
  </Logs>
  <InputFiles>
    <File>gbnk-20140331.xml</File>
    <File>gbnk-20140331.xsd</File>
    <File>gbnk-20140331_cal.xml</File>
    <File>gbnk-20140331_def.xml</File>
    <File>gbnk-20140331_lab.xml</File>
    <File>gbnk-20140331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
