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Income Taxes - Deferred Tax Assets and Liabilities (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Deferred tax assets:    
Loans, principally due to allowance for loan losses $ 28,506us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses $ 30,903us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses
Deferred Costs, Leasing, Gross 719us-gaap_DeferredCostsLeasingGross 0us-gaap_DeferredCostsLeasingGross
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Contingencies 1,785us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsContingencies 0us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsContingencies
Deferred Discounts, Finance Charges and Interest Included in Receivables 7,022us-gaap_DeferredDiscountsFinanceChargesAndInterestIncludedInReceivables 0us-gaap_DeferredDiscountsFinanceChargesAndInterestIncludedInReceivables
Deferred Tax Assets, Unrealized Losses on Trading Securities 2,728us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross 9,473us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross
Employee benefits 8,256us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits 6,813us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits
Other real estate owned write-downs and carrying costs 2,536fibk_DeferredTaxAssetsOtherRealEstateOwnedWriteDownsAndCarryingCosts 4,731fibk_DeferredTaxAssetsOtherRealEstateOwnedWriteDownsAndCarryingCosts
Deferred gain on sale of subsidiary 253fibk_DeferredTaxAssetsDeferredGainSaleOfSubsidiary 484fibk_DeferredTaxAssetsDeferredGainSaleOfSubsidiary
Deferred revenue on contract 481fibk_Deferredrevenueoncontract 545fibk_Deferredrevenueoncontract
Other 760us-gaap_DeferredTaxAssetsOther 299us-gaap_DeferredTaxAssetsOther
Deferred tax assets 53,046us-gaap_DeferredTaxAssetsNet 53,248us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities:    
Fixed assets, principally differences in bases and depreciation (569)us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment (4,227)us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities (1,351)fibk_DeferredTaxLiabilitiesUnrealizedGainsOnAvailableForSaleSecurities 0fibk_DeferredTaxLiabilitiesUnrealizedGainsOnAvailableForSaleSecurities
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (1,053)us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates (700)us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates
Prepaid amounts (1,385)us-gaap_DeferredTaxLiabilitiesPrepaidExpenses (1,247)us-gaap_DeferredTaxLiabilitiesPrepaidExpenses
Government agency stock dividends (2,407)fibk_DeferredTaxLiabilitiesAgencyStockDividends (1,965)fibk_DeferredTaxLiabilitiesAgencyStockDividends
Goodwill and core deposit intangibles (36,732)us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssets (28,167)us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssets
Mortgage servicing rights (4,144)us-gaap_DeferredTaxLiabilitiesMortgageServicingRights (4,337)us-gaap_DeferredTaxLiabilitiesMortgageServicingRights
Other (531)us-gaap_DeferredTaxLiabilitiesOther (451)us-gaap_DeferredTaxLiabilitiesOther
Deferred tax liabilities (48,172)us-gaap_DeferredTaxLiabilities (41,094)us-gaap_DeferredTaxLiabilities
Net deferred tax assets 4,874us-gaap_DeferredTaxAssetsLiabilitiesNet 12,154us-gaap_DeferredTaxAssetsLiabilitiesNet
Income Taxes Payable, Current 4,693us-gaap_TaxesPayableCurrent  
Income Taxes Receivable, Current   $ 2,968us-gaap_IncomeTaxesReceivable