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Employee Benefit Plans (Details Textual) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Employee Benefit Plans |  |  |
Number of officers in employment contract | 25gbci_NumberOfOfficersInEmploymentContract |  |
401(k) and Profit Sharing Plans |  |  |
401(k) and Profit Sharing Plans |  |  |
Minimum age required to be considered eligible for plan | 21gbci_MinimumAgeRequiredToBeConsideredEligibleForPlan / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Period of Employment Required to be Eligible for Plan | 3gbci_PeriodOfEmploymentRequiredToBeEligibleForPlan / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Minimum number of quarters required to be eligible for employer discretionary contribution | 1gbci_MinimumNumberOfQuartersRequiredToBeEligibleForEmployerDiscretionaryContribution / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Minimum number of hours required to be considered eligible for employer discretionary contribution | 501gbci_MinimumNumberOfHoursRequiredToBeConsideredEligibleForEmployerDiscretionaryContribution / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Percent rate for non-elective safe harbor contribution as component of profit sharing plan | 3.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Profit Sharing Plan Expense | $ 7,107,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember | $ 5,862,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember | $ 3,974,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember
Maximum percentage of annual compensation allowable for contribution by eligible employees | 60.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Percentage of amount out of first specific percent of employee's contribution matched under plan | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Percentage of employee's contribution required to determine the matched amount under the plan | 6.00%gbci_PercentageOfEmployeesContributionRequiredToDetermineMatchedAmountUnderPlan / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember |  |
Employee Benefits Plans |  |  |
Company's contribution to the plan | 2,246,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember | 1,935,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember | 1,751,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedContributionPlanNameAxis = gbci_DefinedContributionPlanMember
Deferred Compensation Plans |  |  |
Deferred Compensation Plans |  |  |
Percentage of salary for deferral of cash payments | 50.00%gbci_PercentageOfSalaryForDeferralOfCashPayments / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember |  |
Percentage of bonuses and directors fees for deferral of cash payments | 100.00%gbci_PercentageOfBonusesAndDirectorsFeesForDeferralOfCashPayments / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember |  |
Total amount deferred | 591,000gbci_DeferredAmountForCompensationPlans / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember | 376,000gbci_DeferredAmountForCompensationPlans / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember | 278,000gbci_DeferredAmountForCompensationPlans / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember
Liability related to obligations of deferred compensation plans | 4,810,000us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember | 5,042,000us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember |
Employee Benefits Plans |  |  |
Credit earning rate as a percentage of return on equity | 50.00%gbci_CreditEarningRateAsPercentageOfReturnOnEquity / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember |  |
Total earnings for the plan | 369,000gbci_DefinedContributionPlanEarnings / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember | 515,000gbci_DefinedContributionPlanEarnings / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember | 231,000gbci_DefinedContributionPlanEarnings / us-gaap_DefinedContributionPlanNameAxis = gbci_DeferredCompensationPlanMember
Supplemental Employee Retirement Plan |  |  |
Employee Benefits Plans |  |  |
Company's contribution to the plan | 151,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember | 76,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember | 47,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember
Credit earning rate as a percentage of return on equity | 50.00%gbci_CreditEarningRateAsPercentageOfReturnOnEquity / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember |  |
Total earnings for the plan | $ 59,000gbci_DefinedContributionPlanEarnings / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember | $ 48,000gbci_DefinedContributionPlanEarnings / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember | $ 37,000gbci_DefinedContributionPlanEarnings / us-gaap_DefinedContributionPlanNameAxis = gbci_SupplementalEmployeeRetirementPlansMember

X
- Definition Credit earning rate as a percentage of return on equity. + References No definition available. + Details
Name: | gbci_CreditEarningRateAsPercentageOfReturnOnEquity
Namespace Prefix: | gbci_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Deferred amount for compensation plans. + References No definition available. + Details
Name: | gbci_DeferredAmountForCompensationPlans
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Defined Contribution Plan, Earnings + References No definition available. + Details
Name: | gbci_DefinedContributionPlanEarnings
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | gbci_EmployeeBenefitPlansTextualAbstract
Namespace Prefix: | gbci_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum age required to be considered eligible for plan. + References No definition available. + Details
Name: | gbci_MinimumAgeRequiredToBeConsideredEligibleForPlan
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum Number of Hours Required to be Considered Eligible for Employer Discretionary Contribution of the Profit Sharing Plan. + References No definition available. + Details
Name: | gbci_MinimumNumberOfHoursRequiredToBeConsideredEligibleForEmployerDiscretionaryContribution
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum number of quarters required to be eligible for employer discretionary contribution of the profit sharing plan. + References No definition available. + Details
Name: | gbci_MinimumNumberOfQuartersRequiredToBeEligibleForEmployerDiscretionaryContribution
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of officers in employment contract. + References No definition available. + Details
Name: | gbci_NumberOfOfficersInEmploymentContract
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of bonuses and directors fees for deferral of cash payments. + References No definition available. + Details
Name: | gbci_PercentageOfBonusesAndDirectorsFeesForDeferralOfCashPayments
Namespace Prefix: | gbci_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of employee's contribution required to determine matched amount under plan. + References No definition available. + Details
Name: | gbci_PercentageOfEmployeesContributionRequiredToDetermineMatchedAmountUnderPlan
Namespace Prefix: | gbci_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of salary for deferral of cash payments. + References No definition available. + Details
Name: | gbci_PercentageOfSalaryForDeferralOfCashPayments
Namespace Prefix: | gbci_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period of Employment Required to be Eligible for Plan + References No definition available. + Details
Name: | gbci_PeriodOfEmploymentRequiredToBeEligibleForPlan
Namespace Prefix: | gbci_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DeferredCompensationArrangementsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The carrying amount of the liability as of the balance sheet date to an individual under a deferred compensation arrangement. This amount may be the result of periodic accruals made over the period of active employment, or reflect termination benefits resulting contractual terms or a death benefit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 710 -SubTopic 10 -Section 30 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6409875&loc=d3e20015-108363 + Details
Name: | us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_DefinedContributionPensionAndOtherPostretirementPlansDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
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Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |