XML 25 R79.htm IDEA: XBRL DOCUMENT v2.4.1.9
Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses      
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses $ 287us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses    
Allowance for Loan and Lease Losses [Roll Forward]      
Beginning balance 85,339us-gaap_LoansAndLeasesReceivableAllowance 100,511us-gaap_LoansAndLeasesReceivableAllowance 112,581us-gaap_LoansAndLeasesReceivableAllowance
Provision charged to operating expense (6,622)us-gaap_ProvisionForLoanAndLeaseLosses (6,125)us-gaap_ProvisionForLoanAndLeaseLosses 40,750us-gaap_ProvisionForLoanAndLeaseLosses
Less loans charged-off (13,995)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs (20,513)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs (60,936)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Add back recoveries of loans previously charged-off 9,478us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts 11,466us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts 8,116us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Ending balance 74,200us-gaap_LoansAndLeasesReceivableAllowance 85,339us-gaap_LoansAndLeasesReceivableAllowance 100,511us-gaap_LoansAndLeasesReceivableAllowance
Allowance for loan losses:      
Loans individually evaluated for impairment 5,792us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 8,929us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 10,297us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Loans collectively evaluated for impairment 68,408us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment 76,410us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment 90,214us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Ending balance 74,200us-gaap_LoansAndLeasesReceivableAllowance 85,339us-gaap_LoansAndLeasesReceivableAllowance 100,511us-gaap_LoansAndLeasesReceivableAllowance
Total loans:      
Individually evaluated for impairment 78,480us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment 110,572us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment 136,185us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Individually evaluated for impairment 4,818,963us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment 4,234,281us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment 4,087,727us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Total Loans 4,897,443fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment 4,344,853fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment 4,223,912fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
Real Estate      
Allowance for Loan and Lease Losses [Roll Forward]      
Beginning balance 63,923us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
75,782us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
87,396us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Provision charged to operating expense (10,348)us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
(7,722)us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
28,651us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Less loans charged-off (3,014)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
(10,224)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
(43,506)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Add back recoveries of loans previously charged-off 3,323us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
6,087us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
3,241us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Ending balance 53,884us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
63,923us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
75,782us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Allowance for loan losses:      
Loans individually evaluated for impairment 3,961us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
7,339us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
8,350us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Loans collectively evaluated for impairment 49,923us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
56,584us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
67,432us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Ending balance 53,884us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
63,923us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
75,782us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Total loans:      
Individually evaluated for impairment 62,775us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
96,862us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
123,406us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Individually evaluated for impairment 3,203,306us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
2,786,254us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
2,660,420us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Total Loans 3,266,081fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
2,883,116fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
2,783,826fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_RealEstatePortfolioSegmentMember
Consumer      
Allowance for Loan and Lease Losses [Roll Forward]      
Beginning balance 6,193us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
7,141us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
8,594us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Provision charged to operating expense 1,382us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
1,605us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
1,922us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Less loans charged-off (4,887)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
(4,612)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
(5,320)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Add back recoveries of loans previously charged-off 2,347us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
2,059us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
1,945us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Ending balance 5,035us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
6,193us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
7,141us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Allowance for loan losses:      
Loans individually evaluated for impairment 0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Loans collectively evaluated for impairment 5,035us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
6,193us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
7,141us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Ending balance 5,035us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
6,193us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
7,141us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Total loans:      
Individually evaluated for impairment 0us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
0us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
0us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Individually evaluated for impairment 762,471us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
671,587us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
636,794us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Total Loans 762,471fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
671,587fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
636,794fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Commercial      
Allowance for Loan and Lease Losses [Roll Forward]      
Beginning balance 14,747us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
17,085us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
15,325us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Provision charged to operating expense 1,809us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
41us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
10,845us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Less loans charged-off (6,030)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
(5,672)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
(11,990)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Add back recoveries of loans previously charged-off 3,781us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
3,293us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
2,905us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Ending balance 14,307us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
14,747us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
17,085us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Allowance for loan losses:      
Loans individually evaluated for impairment 1,190us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
1,504us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
1,919us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Loans collectively evaluated for impairment 13,117us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
13,243us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
15,166us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Ending balance 14,307us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
14,747us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
17,085us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Total loans:      
Individually evaluated for impairment 14,526us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
13,585us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
12,242us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Individually evaluated for impairment 725,547us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
662,959us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
676,511us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Total Loans 740,073fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
676,544fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
688,753fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Agriculture      
Allowance for Loan and Lease Losses [Roll Forward]      
Beginning balance 476us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
503us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
1,266us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Provision charged to operating expense 535us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
(49)us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
(668)us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Less loans charged-off (64)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
(5)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
(120)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Add back recoveries of loans previously charged-off 27us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
27us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
25us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Ending balance 974us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
476us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
503us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Allowance for loan losses:      
Loans individually evaluated for impairment 641us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
86us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
28us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Loans collectively evaluated for impairment 333us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
390us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
475us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Ending balance 974us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
476us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
503us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Total loans:      
Individually evaluated for impairment 1,179us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
125us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
537us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Individually evaluated for impairment 123,680us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
111,747us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
113,090us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Total Loans 124,859fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
111,872fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
113,627fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_AgriculturePortfolioSegmentMember
Other      
Allowance for Loan and Lease Losses [Roll Forward]      
Beginning balance 0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Provision charged to operating expense 0us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_ProvisionForLoanAndLeaseLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Less loans charged-off 0us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Add back recoveries of loans previously charged-off 0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Ending balance 0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Allowance for loan losses:      
Loans individually evaluated for impairment 0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Loans collectively evaluated for impairment 0us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Ending balance 0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Total loans:      
Individually evaluated for impairment 0us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
0us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Individually evaluated for impairment 3,959us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
1,734us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
912us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
Total Loans $ 3,959fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
$ 1,734fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember
$ 912fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fibk_OtherPortfolioSegmentMember