XML 66 R53.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Nature of Operations and Summary of Significant Accounting Policies (Details Textual) | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) quarter step reporting_unit division operating_segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Number of Bank Divisions | division | 14 |  |
Number of Operating Segments | operating_segment | 1 |  |
Maximum Period of Original Maturity to be Included in Cash and Cash Equivalents | 3 months |  |
Federal Reserve Balance or Cash on Hand Required | $ 53,626,000 |  |
Minimum Period Past Due to Consider Loans as Delinquent | 30 days |  |
Minimum Period Past Due to Consider Loans as Non Accrual | 90 days |  |
Number of Quarters Used to Evaluate Historical Loss Experience | quarter | 12 |  |
Maximum period up to which purchase price of business is allocated to assets acquired and liabilities assumed | 1 year |  |
Number of Reporting Units Subsequent to Aggregation for Goodwill Assessment | reporting_unit | 1 |  |
Number of steps included in goodwill impairment test | step | 2 |  |
Revenue from Contracts with Customers | $ 76,664,000 | $ 69,808,000 | $ 64,722,000
Impairment losses on receivables related to contracts with customers | $ 0 | 0 |
Percentage of likelihood for term more likely than not as it relates to income taxes | 50.00% |  |
Transfer of debt securities from held-to-maturity to available-for-sale | $ 270,331,000 | $ 0 | $ 0
Accounting Standards Update 2017-12 |  |  |
Transfer of debt securities from held-to-maturity to available-for-sale | $ 270,331,000 |  |
Accounting Standards Update 2014-09 |  |  |
Number of Steps to Achieve Core Revenue Recognition Principal | step | 5 |  |
Accounting Standards Update 2017-08 |  |  |
Effect of cumulative change on retained earnings from applying new accounting pronouncement | $ 24,102,000 |  |
Accounting Standards Update 2016-02 |  |  |
Effect of cumulative change on assets from applying new accounting pronouncement | 36,042,000 |  |
Effect of cumulative change on liabilities from applying new accounting pronouncement | $ 37,384,000 |  |
Consumer and Other |  |  |
Minimum Number of Days Delinquent to Charge Off Loans | 120 days |  |
Minimum Range |  |  |
Number of years for home equity loan origination term | 10 years |  |
Minimum Range | Office Buildings |  |  |
Useful life of premises and equipment | 15 years |  |
Minimum Range | Furniture, fixtures and equipment |  |  |
Useful life of premises and equipment | 3 years |  |
Maximum Range |  |  |
Number of years for home equity loan origination term | 15 years |  |
Maximum Range | Office Buildings |  |  |
Useful life of premises and equipment | 40 years |  |
Maximum Range | Furniture, fixtures and equipment |  |  |
Useful life of premises and equipment | 10 years |  |

X
- Definition Federal Reserve balance or cash on hand required. + References No definition available. + Details
Name: | gbci_FederalReserveBalanceOrCashOnHandRequired
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The number of quarters used to evaluate the historical loss experience when determining the general valuation allowance component of the allowance for credit losses. + References No definition available. + Details
Name: | gbci_FinancingReceivableAllowanceforCreditLossesNumberofQuartersUsedtoEvaluateHistoricalLossExperience
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum Period up to Which Purchase Price of Business is Allocated to Assets Acquired and Liabilities Assumed. + References No definition available. + Details
Name: | gbci_MaximumPeriodUpToWhichPurchasePriceOfBusinessIsAllocatedToAssetsAcquiredAndLiabilitiesAssumed
Namespace Prefix: | gbci_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum Period of Original Maturity to be Included in Cash and Cash Equivalents + References No definition available. + Details
Name: | gbci_MaximumPeriodofOriginalMaturitytobeIncludedinCashandCashEquivalents
Namespace Prefix: | gbci_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum Period Past Due to Consider Loans as Delinquent + References No definition available. + Details
Name: | gbci_MinimumPeriodPastDuetoConsiderLoansasDelinquent
Namespace Prefix: | gbci_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum Period Past Due to Consider Loans as Non Accrual + References No definition available. + Details
Name: | gbci_MinimumPeriodPastDuetoConsiderLoansasNonAccrual
Namespace Prefix: | gbci_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Assets + References No definition available. + Details
Name: | gbci_NewAccountingPronouncementorChangeinAccountingPrincipleCumulativeEffectofChangeonAssets
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Liabilities + References No definition available. + Details
Name: | gbci_NewAccountingPronouncementorChangeinAccountingPrincipleCumulativeEffectofChangeonLiabilities
Namespace Prefix: | gbci_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number of Bank Divisions + References No definition available. + Details
Name: | gbci_NumberOfBankDivisions
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Reporting Units Subsequent to Aggregation for Goodwill Assessment + References No definition available. + Details
Name: | gbci_NumberOfReportingUnitsSubsequentToAggregationForGoodwillAssessment
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Steps to Achieve Core Revenue Recognition Principal + References No definition available. + Details
Name: | gbci_NumberofStepstoAchieveCoreRevenueRecognitionPrincipal
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of steps included in goodwill impairment test + References No definition available. + Details
Name: | gbci_Numberofstepsincludedingoodwillimpairmenttest
Namespace Prefix: | gbci_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of years for home equity loan origination term + References No definition available. + Details
Name: | gbci_Numberofyearsforhomeequityloanoriginationterm
Namespace Prefix: | gbci_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of likelihood for term more likely than not. + References No definition available. + Details
Name: | gbci_PercentageOfLikelihoodForTermMoreLikelyThanNot
Namespace Prefix: | gbci_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of accumulated impairment loss for asset recognized from cost incurred to obtain or fulfill contract with customer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054 + Details
Name: | us-gaap_CapitalizedContractCostAccumulatedImpairment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22580-107794 + Details
Name: | us-gaap_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnEquityOrNetAssets1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045 + Details
Name: | us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Threshold period past due for financing receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_ThresholdPeriodPastDueForWriteoffOfFinancingReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Value of investments transferred from the entity's investments in noncash transactions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
Name: | us-gaap_TransferOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | gbci_AdjustmentsDuetoNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201712Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | gbci_AdjustmentsDuetoNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201409Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | gbci_AdjustmentsDuetoNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201708Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | gbci_AdjustmentsDuetoNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201602Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |