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Consideration Transferred, Identifiable Net Assets Acquired and Goodwill Recognized (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Liabilities assumed      
Bargain purchase gain $ (680)us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount $ 0us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount $ 0us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount
Goodwill recognized 0us-gaap_GoodwillAcquiredDuringPeriod 23,606us-gaap_GoodwillAcquiredDuringPeriod 0us-gaap_GoodwillAcquiredDuringPeriod
FNBR Holding Corporation      
Fair value of consideration transferred      
Fair value of Company shares issued, net of equity issuance costs 15,127us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Cash consideration for outstanding shares 16,690us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Contingent consideration 0gbci_BusinessCombinationContingentConsideration
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Total fair value of consideration transferred 31,817us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Identifiable assets acquired      
Cash and cash equivalents 14,578us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Investment securities 157,018gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Loans receivable 137,488gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedLoansReceivable
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Core deposit intangible 4,199us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Accrued income and other assets 35,884gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherAssets
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Total identifiable assets acquired 349,167us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Liabilities assumed      
Deposits 309,641gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDeposits
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Federal Home Loan Bank advances 0gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedFederalHomeLoanBankAdvances
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Accrued expenses and other liabilities 7,029gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherLiabilities
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Total liabilities assumed 316,670us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Total identifiable net assets 32,497us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
Bargain purchase gain (680)us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount
/ us-gaap_BusinessAcquisitionAxis
= gbci_FNBRHoldingCorporationMember
   
North Cascades Bancshares      
Fair value of consideration transferred      
Fair value of Company shares issued, net of equity issuance costs   16,743us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Cash consideration for outstanding shares   13,833us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Contingent consideration   0gbci_BusinessCombinationContingentConsideration
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Total fair value of consideration transferred   30,576us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Identifiable assets acquired      
Cash and cash equivalents   27,865us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Investment securities   48,058gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Loans receivable   215,986gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedLoansReceivable
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Core deposit intangible   3,660us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Accrued income and other assets   24,262gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherAssets
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Total identifiable assets acquired   319,831us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Liabilities assumed      
Deposits   294,980gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDeposits
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Federal Home Loan Bank advances   0gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedFederalHomeLoanBankAdvances
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Accrued expenses and other liabilities   4,472gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherLiabilities
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Total liabilities assumed   299,452us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Total identifiable net assets   20,379us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Goodwill recognized   10,197us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_BusinessAcquisitionAxis
= gbci_NorthCascadesBancsharesMember
 
Wheatland Bankshares      
Fair value of consideration transferred      
Fair value of Company shares issued, net of equity issuance costs   28,290us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Cash consideration for outstanding shares   11,025us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Contingent consideration   0gbci_BusinessCombinationContingentConsideration
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Total fair value of consideration transferred   39,315us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Identifiable assets acquired      
Cash and cash equivalents   23,148us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Investment securities   75,643gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Loans receivable   171,199gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedLoansReceivable
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Core deposit intangible   2,079us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Accrued income and other assets   15,063gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherAssets
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Total identifiable assets acquired   287,132us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Liabilities assumed      
Deposits   255,197gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDeposits
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Federal Home Loan Bank advances   5,467gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedFederalHomeLoanBankAdvances
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Accrued expenses and other liabilities   562gbci_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherLiabilities
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Total liabilities assumed   261,226us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Total identifiable net assets   25,906us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember
 
Goodwill recognized   $ 13,409us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_BusinessAcquisitionAxis
= gbci_WheatlandBanksharesMember