XML 81 R71.htm IDEA: XBRL DOCUMENT
v3.19.2

Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - Significant Unobservable Inputs (Level 3) - Fair Value Measured on a Non-recurring Basis $ in Millions | Jun. 30, 2019 USD ($) | Dec. 31, 2018 USD ($)
--- | --- | ---
Carrying Amount |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Impaired loans | $ 38.9 | $ 36.3
Estimated Fair Value |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Impaired loans | 26.2 | 24.1
Impaired loans |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Impaired loans |  | 24.1
Impaired loans | Estimated Fair Value |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Impaired loans | $ 26.2 | 24.1
Impaired loans | Discount Rate | Minimum | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0 |
Impaired loans | Discount Rate | Maximum | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.26 |
Impaired loans | Discount Rate | Weighted Average | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.13 |
Other real estate owned |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Other real estate owned | $ 2.1 | 0.6
Other real estate owned | Estimated Fair Value |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Other real estate owned | $ 2.1 | 0.6
Other real estate owned | Discount Rate | Minimum | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.08 |
Other real estate owned | Discount Rate | Maximum | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.96 |
Other real estate owned | Discount Rate | Weighted Average | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.39 |
Long-lived assets to be disposed of by sale |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Long-lived assets to be disposed of by sale | $ 5.0 | 4.9
Long-lived assets to be disposed of by sale | Estimated Fair Value |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Long-lived assets to be disposed of by sale | $ 5.0 | $ 4.9
Long-lived assets to be disposed of by sale | Discount Rate | Minimum | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0 |
Long-lived assets to be disposed of by sale | Discount Rate | Maximum | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.43 |
Long-lived assets to be disposed of by sale | Discount Rate | Weighted Average | Market Approach Valuation Technique |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Discount rate | 0.10 |

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 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=99377789&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: |