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Derivatives and Hedging Activities (Details Textual) (USD $)
3 Months Ended
Mar. 31, 2015
Interest Rate Swap  
Interest rate swap derivative financial instruments  
Investment securities pledged as collateral $ 27,133,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
Collateral pledged from the counterparties to the Company 0us-gaap_SecuritiesReceivedAsCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
Interest Rate Swap One  
Interest rate swap derivative financial instruments  
Forecasted Notional Amount 160,000,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gbci_InterestRateSwapOneMember
Interest expense recorded on interest rate swap 1,351,000us-gaap_DerivativeCostOfHedge
/ us-gaap_DerivativeByNatureAxis
= gbci_InterestRateSwapOneMember
Unrealized loss to be reclassified within twelve months $ 5,064,000us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
/ us-gaap_DerivativeByNatureAxis
= gbci_InterestRateSwapOneMember