XML 58 R47.htm IDEA: XBRL DOCUMENT
v3.24.2.u1

Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | $ 2,799.2 | $ 3,208.7
AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 2,632.9 | 3,041.5
AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 91.1 | 92.3
A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 10.2 | 10.6
BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 47.0 | 47.0
Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 13.0 | 12.3
BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 5.0 | 5.0
U.S. Treasury notes |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 99.5 | 399.0
U.S. Treasury notes | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 99.5 | 399.0
U.S. Treasury notes | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
U.S. Treasury notes | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
U.S. Treasury notes | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
U.S. Treasury notes | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
U.S. Treasury notes | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
State, county, and municipal securities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 178.2 | 179.2
State, county, and municipal securities | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 68.9 | 69.0
State, county, and municipal securities | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 91.1 | 92.3
State, county, and municipal securities | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 10.2 | 10.6
State, county, and municipal securities | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
State, county, and municipal securities | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 8.0 | 7.3
State, county, and municipal securities | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Obligations of U.S. government agencies |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 356.0 | 354.5
Obligations of U.S. government agencies | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 356.0 | 354.5
Obligations of U.S. government agencies | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Obligations of U.S. government agencies | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Obligations of U.S. government agencies | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Obligations of U.S. government agencies | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Obligations of U.S. government agencies | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Corporate securities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 57.0 | 57.0
Corporate securities | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Corporate securities | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Corporate securities | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Corporate securities | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 47.0 | 47.0
Corporate securities | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 5.0 | 5.0
Corporate securities | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 5.0 | 5.0
Commercial Mortgage-Backed Securities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 496.2 | 510.5
Commercial Mortgage-Backed Securities | FNMA/FHLMC |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 350.9 | 359.7
Commercial Mortgage-Backed Securities | FNMA/FHLMC | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 350.9 | 359.7
Commercial Mortgage-Backed Securities | FNMA/FHLMC | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | GNMA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 145.3 | 150.8
Commercial Mortgage-Backed Securities | GNMA | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 145.3 | 150.8
Commercial Mortgage-Backed Securities | GNMA | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | GNMA | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | GNMA | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | GNMA | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Commercial Mortgage-Backed Securities | GNMA | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 1,159.6 | 1,232.6
Residential Mortgage Backed Securities | FNMA/FHLMC |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 1,119.4 | 1,189.8
Residential Mortgage Backed Securities | FNMA/FHLMC | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 1,119.4 | 1,189.8
Residential Mortgage Backed Securities | FNMA/FHLMC | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | GNMA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 40.2 | 42.8
Residential Mortgage Backed Securities | GNMA | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 40.2 | 42.8
Residential Mortgage Backed Securities | GNMA | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | GNMA | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | GNMA | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | GNMA | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Residential Mortgage Backed Securities | GNMA | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 452.7 | 475.9
Collateralized Mortgage Obligations | FNMA/FHLMC |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 317.2 | 334.1
Collateralized Mortgage Obligations | FNMA/FHLMC | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 317.2 | 334.1
Collateralized Mortgage Obligations | FNMA/FHLMC | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | FNMA/FHLMC | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | FNMA/FHLMC | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | FNMA/FHLMC | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | FNMA/FHLMC | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | GNMA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 135.5 | 141.8
Collateralized Mortgage Obligations | GNMA | AAA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 135.5 | 141.8
Collateralized Mortgage Obligations | GNMA | AA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | GNMA | A |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | GNMA | BBB |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | GNMA | Not Rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | 0.0 | 0.0
Collateralized Mortgage Obligations | GNMA | BB Rating |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Investment securities classified as held to maturity | $ 0.0 | $ 0.0

X
- Definition Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479344/326-20-45-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-14 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 7A -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-7A + Details
Name: | us-gaap_HeldToMaturitySecurities
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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (aaa) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5A Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5A Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5A Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5B -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5B Reference 14: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5B Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5B -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5B Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5B -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5B Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5B -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5B Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 + Details
Name: | us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | fibk_CreditRatingAxis=fibk_AAARatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_CreditRatingAxis=fibk_AARatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_CreditRatingAxis=fibk_ARatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_CreditRatingAxis=fibk_BBBRatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_CreditRatingAxis=fibk_NotRatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_CreditRatingAxis=fibk_BBRatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

X
- Details
Name: | us-gaap_InvestmentTypeAxis=fibk_FNMAAndFHLMCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=fibk_GNMAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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