XML 51 R58.htm IDEA: XBRL DOCUMENT
v3.19.3

Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | $ 8,992.6 | $ 8,470.4
Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 3,468.2 | 3,235.4
Land acquisition & development |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 309.6 | 321.6
Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 261.8 | 242.8
Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 386.7 | 274.3
Total construction loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 958.1 | 838.7
Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1,556.3 | 1,542.0
Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 226.5 | 217.4
Total real estate loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 6,209.1 | 5,833.5
Indirect consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 797.8 | 787.8
Direct consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 188.8 | 200.6
Total consumer loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1,066.9 | 1,070.2
Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1,422.6 | 1,310.3
Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 292.7 | 254.8
Other Assets Especially Mentioned |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 174.5 | 178.8
Other Assets Especially Mentioned | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 90.0 | 102.5
Other Assets Especially Mentioned | Land acquisition & development |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 5.0 | 5.0
Other Assets Especially Mentioned | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.9 | 2.8
Other Assets Especially Mentioned | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 2.3 | 1.7
Other Assets Especially Mentioned | Total construction loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 8.2 | 9.5
Other Assets Especially Mentioned | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 3.3 | 3.0
Other Assets Especially Mentioned | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 15.5 | 9.0
Other Assets Especially Mentioned | Total real estate loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 117.0 | 124.0
Other Assets Especially Mentioned | Indirect consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.3 | 0.7
Other Assets Especially Mentioned | Direct consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.4 | 0.3
Other Assets Especially Mentioned | Total consumer loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.7 | 1.0
Other Assets Especially Mentioned | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 40.2 | 39.4
Other Assets Especially Mentioned | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 16.6 | 14.4
Substandard |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 229.1 | 201.6
Substandard | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 96.5 | 87.4
Substandard | Land acquisition & development |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 5.0 | 7.0
Substandard | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1.4 | 2.0
Substandard | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1.5 | 3.9
Substandard | Total construction loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 7.9 | 12.9
Substandard | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 7.0 | 10.8
Substandard | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 25.3 | 24.0
Substandard | Total real estate loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 136.7 | 135.1
Substandard | Indirect consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 3.2 | 2.1
Substandard | Direct consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1.0 | 0.8
Substandard | Total consumer loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 4.2 | 2.9
Substandard | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 62.5 | 45.8
Substandard | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 25.7 | 17.8
Doubtful |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 12.5 | 20.9
Doubtful | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1.5 | 2.9
Doubtful | Land acquisition & development |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.1 | 3.3
Doubtful | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.4 | 0.4
Doubtful | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.0 | 0.0
Doubtful | Total construction loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.5 | 3.7
Doubtful | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.3 | 0.7
Doubtful | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.0 | 0.1
Doubtful | Total real estate loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 2.3 | 7.4
Doubtful | Indirect consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.0 | 0.1
Doubtful | Direct consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.1 | 0.1
Doubtful | Total consumer loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.1 | 0.2
Doubtful | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 9.9 | 11.8
Doubtful | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 0.2 | 1.5
Total Criticized Loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 416.1 | 401.3
Total Criticized Loans | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 188.0 | 192.8
Total Criticized Loans | Land acquisition & development |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 10.1 | 15.3
Total Criticized Loans | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 2.7 | 5.2
Total Criticized Loans | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 3.8 | 5.6
Total Criticized Loans | Total construction loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 16.6 | 26.1
Total Criticized Loans | Residential |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 10.6 | 14.5
Total Criticized Loans | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 40.8 | 33.1
Total Criticized Loans | Total real estate loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 256.0 | 266.5
Total Criticized Loans | Indirect consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 3.5 | 2.9
Total Criticized Loans | Direct consumer |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 1.5 | 1.2
Total Criticized Loans | Total consumer loans |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 5.0 | 4.1
Total Criticized Loans | Commercial |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | 112.6 | 97.0
Total Criticized Loans | Agricultural |  |
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items] |  |
Loans held for investment | $ 42.5 | $ 33.7

X
- Definition Gross carrying amount of all categories of loans and leases held in the portfolio, excluding loans held for sale. + References No definition available. + Details
Name: | fibk_LoansAndLeasesReceivableHeldForInvestment
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_LandAcquisitionAndDevelopmentConstructionFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_ResidentialRealEstateConstructionFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_CommercialRealEstateConstructionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_RealEstateConstructionFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_AgriculturalRealEstateFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_RealEstateFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_ConsumerIndirectFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fibk_AgriculturalFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=fibk_TotalCriticizedLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |