XML 76 R66.htm IDEA: XBRL DOCUMENT
v3.20.2

Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Material write downs | $ 0.1 | $ 0.2 | $ 0.1 | $ 0.7 |
Significant Unobservable Inputs (Level 3) | Carrying Amount | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Collateral dependent loans* | 16.0 |  | 16.0 |  | $ 41.3
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Collateral dependent loans* | 11.4 |  | 11.4 |  | 27.6
Collateral dependent loans | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Collateral dependent loans* |  |  |  |  | 27.6
Collateral dependent loans | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Collateral dependent loans* | $ 11.4 |  | $ 11.4 |  | 27.6
Collateral dependent loans | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Minimum | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0 |  | 0 |  |
Collateral dependent loans | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Maximum | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.56 |  | 0.56 |  |
Collateral dependent loans | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Weighted Average | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.22 |  | 0.22 |  |
Other real estate owned | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Other real estate owned | $ 1.1 |  | $ 1.1 |  | 2.2
Other real estate owned | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Other real estate owned | $ 1.1 |  | $ 1.1 |  | 2.2
Other real estate owned | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Minimum | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.08 |  | 0.08 |  |
Other real estate owned | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Maximum | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.65 |  | 0.65 |  |
Other real estate owned | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Weighted Average | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.27 |  | 0.27 |  |
Long-lived assets to be disposed of by sale | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Long-lived assets to be disposed of by sale | $ 5.0 |  | $ 5.0 |  | 6.2
Long-lived assets to be disposed of by sale | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Long-lived assets to be disposed of by sale | $ 5.0 |  | $ 5.0 |  | $ 6.2
Long-lived assets to be disposed of by sale | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Minimum | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0 |  | 0 |  |
Long-lived assets to be disposed of by sale | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Maximum | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.37 |  | 0.37 |  |
Long-lived assets to be disposed of by sale | Discount Rate | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis | Weighted Average | Market Approach Valuation Technique |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |  |  |
Discount rate | 0.03 |  | 0.03 |  |

X
- Definition Loans Receivable, Impaired, Fair Value Disclosure + References No definition available. + Details
Name: | fibk_LoansReceivableImpairedFairValueDisclosure
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Long-Lived Assets to be Disposed, Fair Value Disclosure + References No definition available. + Details
Name: | fibk_LongLivedAssetsToBeDisposedFairValueDisclosure
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Real Estate Acquired Through Foreclosure, Fair Value Disclosure + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureFairValueDisclosure
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Real Estate Acquired Through Foreclosure, Valuation Adjustments + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureValuationAdjustments
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Value of input used to measure servicing asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_ServicingAssetMeasurementInput
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fibk_LoansAndLeasesReceivableImpairedLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fibk_LoansAndLeasesReceivableOtherRealEstateOwnedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |