XML 102 R81.htm IDEA: XBRL DOCUMENT
v3.22.0.1

Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 16, 2018 USD ($) | Dec. 31, 2007 USD ($) | Dec. 31, 2021 USD ($) trusts | Dec. 31, 2010 | Dec. 31, 2020 USD ($) | May 31, 2020 | Jan. 31, 2008 USD ($) | Nov. 30, 2007 | Oct. 31, 2007 USD ($)
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Number of company sponsored wholly-owned business trusts | trusts |  |  | 7 |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 87.0 |  | $ 87.0 |  |  |  |
Debt instrument, interest rate at period end |  |  | 2.88% |  |  |  |  |  |
Stated interest rate |  |  |  |  |  | 5.25% |  |  |
Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  | 6.78% |  |
LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  |  |  | 2.75% |  |  |  |  |
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 10.3 |  | 10.3 |  |  |  | $ 10.3
Debt instrument, interest rate at period end |  |  | 2.38% |  |  |  |  |  |
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.25% |  |  |  |  |  |  |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 15.5 |  | 15.5 |  |  |  | $ 15.5
Debt instrument, interest rate at period end |  |  | 2.95% |  |  |  |  |  |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  | 7.50% |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.75% |  |  |  |  |  |  |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  | $ 20.6 | $ 20.6 |  | 20.6 |  |  |  |
Debt instrument, interest rate at period end |  |  | 2.60% |  |  |  |  |  |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Stated interest rate |  | 6.88% |  |  |  |  |  |  |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.40% |  |  |  |  |  |  |
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 15.5 |  | 15.5 |  |  |  |
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 10.3 |  | 10.3 |  | $ 10.3 |  |
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  |  | 2.83% |  |  |  |  |  |
Debt instrument, interest rate at period end |  | 2.70% |  |  |  |  |  |  |
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  | $ 15.5 | $ 10.3 |  | 10.3 |  | $ 10.3 |  |
Debt instrument, basis spread on variable rate |  |  | 2.88% | 2.75% |  |  |  |  |
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 4.5 |  | $ 4.5 |  |  |  |
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate | 1.70% |  |  |  |  |  |  |  |
Debt instrument, interest rate at period end |  |  | 1.92% |  |  |  |  |  |
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member] | Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Stated interest rate | 5.95% |  |  |  |  |  |  |  |
Trust Preferred Securities Subject to Mandatory Redemption | Trusts |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption |  |  | $ 84.2 |  |  |  |  |  |
Business trust term (in years) |  |  | 30 years |  |  |  |  |  |
Trust Preferred Securities Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust 1 [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption | $ 5.0 |  |  |  |  |  |  |  |
Common Stock Subject to Mandatory Redemption | Trusts |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption |  |  | $ 2.8 |  |  |  |  |  |
Common Stock Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust I [Member] |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption | $ 0.2 |  |  |  |  |  |  |  |
Subordinated Debentures Subject to Mandatory Redemption | Trusts |  |  |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |  |  |
Business trust term (in years) |  |  | 30 years |  |  |  |  |  |

X
- Definition Company Sponsored Wholly-Owned Business Trusts, Number + References No definition available. + Details
Name: | fibk_CompanySponsoredWhollyOwnedBusinessTrustsNumber
Namespace Prefix: | fibk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Financial Instruments Subject To Mandatory Redemption, Settlement Terms, Term + References No definition available. + Details
Name: | fibk_FinancialInstrumentsSubjectToMandatoryRedemptionSettlementTermsTerm
Namespace Prefix: | fibk_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123466204&loc=SL6036836-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=124435984&loc=d3e28551-108399 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124429444&loc=SL124452920-239629 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 1B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
Name: | us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount that would be paid, determined under the conditions specified in the contract, if the holder of the share has the right to redeem the shares. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109262807&loc=d3e22047-110879 + Details
Name: | us-gaap_SharesSubjectToMandatoryRedemptionSettlementTermsAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_SubordinatedBorrowingLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DebtInstrumentRedemptionPeriodAxis=fibk_DebtInstrumentRedemptionAtIssuanceThroughYearFiveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentRedemptionPeriodAxis=fibk_DebtInstrumentRedemptionAfterPeriodFiveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_FirstInterstateStatutoryTrustIiMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedByFistIiMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_FirstInterstateStatutoryTrustIFistMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedByFistIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_FirstInterstateStatutoryTrustIiiMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedByFistIiiMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_FirstInterstateStatutoryTrustIvMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedByFistIvMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_FirstInterstateStatutoryTrustVMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedByFistVMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_FirstInterstateStatutoryTrustViMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedByFistViMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_NorthwestBancorporationCapitalTrust1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_NorthwestBancorporationCapitalTrustIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=fibk_JuniorSubordinatedDeferrableInterestDebenturesIssuedbyNWBCIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis=us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fibk_TrustsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis=us-gaap_CommonStockSubjectToMandatoryRedemptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis=us-gaap_SubordinatedDebenturesSubjectToMandatoryRedemptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |