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Loans Schedule of Loans Acquired from Business Combinations (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Deteriorated Loans Transferred in [Abstract]    
Outstanding Balance $ 41,910us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesOutstandingBalance $ 0us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesOutstandingBalance
Loans on Accrual Status 31,870fibk_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingValueAccrualStatus 0fibk_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingValueAccrualStatus
Loans on Non-Accrual Status 0fibk_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingValueNonAccrualStatus 0fibk_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingValueNonAccrualStatus
Total Carrying Value $ 31,870us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet $ 0us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet