XML 81 R71.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Write downs on OREO properties | $ 0 | $ 0 |
Minimum | Market Approach Valuation Technique |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Discount Rate for Level 3 Fair Value Disclosure | 0.00% |  |
Maximum | Market Approach Valuation Technique |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Discount Rate for Assets HFS Level 3 Fair Value | 0.00% |  |
Discount Rate for OREO Level 3 Fair Value | 96.20% |  |
Weighted Average | Market Approach Valuation Technique |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Discount Rate for Assets HFS Level 3 Fair Value | 0.00% |  |
Discount Rate for OREO Level 3 Fair Value | 11.60% |  |
Discount Rate for Level 3 Fair Value Disclosure | 26.10% |  |
Carrying Amount |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Net loans held for investment | $ 8,480,300,000 |  | $ 7,542,200,000
Change During Period | Fair Value Measured on a Non-recurring Basis |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Fair Value of long-lived assets to be disposed of by sale | 500,000 |  | 0
Estimated Fair Value |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Net loans held for investment | 8,406,800,000 |  | 7,298,800,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Impaired loans | 28,700,000 |  | 32,600,000
Other real estate owned | 600,000 |  | 1,300,000
Fair Value of long-lived assets to be disposed of by sale | 3,500,000 |  | 800,000
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) |  |  |
Impaired loans |  |  | 32,600,000
Other real estate owned | 600,000 |  | 1,300,000
Fair Value of long-lived assets to be disposed of by sale | 3,500,000 |  | 800,000
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) |  |  |
Impaired loans | 43,500,000 |  | 54,800,000
Fair Value of long-lived assets to be disposed of by sale | 4,000,000 |  | 800,000
Significant Unobservable Inputs (Level 3) | Estimated Fair Value |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Net loans held for investment |  |  | 32,600,000
Impaired loans | 28,700,000 |  |
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) |  |  |
Impaired loans | $ 28,700,000 |  | $ 32,600,000
Measurement Input, Discount Rate [Member] | Minimum | Market Approach Valuation Technique |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Discount Rate for Assets HFS Level 3 Fair Value | 0.00% |  |
Discount Rate for OREO Level 3 Fair Value | 8.00% |  |
Measurement Input, Discount Rate [Member] | Maximum | Market Approach Valuation Technique |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |  |
Discount Rate for Level 3 Fair Value Disclosure | 78.10% |  |

X
- Definition Discount Rate for Assets HFS Level 3 Fair Value + References No definition available. + Details
Name: | fibk_DiscountRateforAssetsHFSLevel3FairValue
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Discount Rate for Impaired Loans Level 3 Fair Value + References No definition available. + Details
Name: | fibk_DiscountRateforLevel3FairValueDisclosure
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Discount Rate for OREO Level 3 Fair Value + References No definition available. + Details
Name: | fibk_DiscountRateforOREOLevel3FairValue
Namespace Prefix: | fibk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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 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 Fair value portion of assets classified as held for sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19190-110258 + Details
Name: | us-gaap_AssetsHeldForSaleLongLivedFairValueDisclosure
Namespace Prefix: | us-gaap_
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 Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772 + Details
Name: | us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of write-down or reserve recognized in the income statement on real estate investments for entities with a substantial portion of business acquiring and holding investment real estate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 970 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-28(Footnote 5)) -URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024 + Details
Name: | us-gaap_SECScheduleIIIRealEstateWritedownOrReserveAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementBasisAxis=us-gaap_ChangeDuringPeriodFairValueDisclosureMember
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_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
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: |