XML 79 R66.htm IDEA: XBRL DOCUMENT
v3.22.4

Acquisitions - Schedule of Adjustments to Goodwill (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2022 USD ($)
Business Acquisition [Line Items] |
Change in goodwill | $ (36.7)
Accrued Expenses and Other Liabilities |
Business Acquisition [Line Items] |
Change in goodwill | 3.3
Total Liability Adjustments |
Business Acquisition [Line Items] |
Change in goodwill | 3.3
Mortgage loans originated for sale |
Business Acquisition [Line Items] |
Change in goodwill | 35.1
Loans held for investment |
Business Acquisition [Line Items] |
Change in goodwill | (8.4)
Allowance for Credit Losses |
Business Acquisition [Line Items] |
Change in goodwill | 24.8
Premises and Equipment |
Business Acquisition [Line Items] |
Change in goodwill | 0.6
Core Deposit Intangibles |
Business Acquisition [Line Items] |
Change in goodwill | 1.0
Deferred Tax Assets |
Business Acquisition [Line Items] |
Change in goodwill | (15.9)
Total Asset Adjustments |
Business Acquisition [Line Items] |
Change in goodwill | 40.0
Other Assets |
Business Acquisition [Line Items] |
Change in goodwill | $ 2.8

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

X
- Definition Amount of increase (decrease) of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267 + Details
Name: | us-gaap_GoodwillPeriodIncreaseDecrease
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | fibk_LiabilityAdjustmentsAxis=fibk_AccruedExpensesAndOtherLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_LiabilityAdjustmentsAxis=fibk_TotalLiabilityAdjustmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_MortgageLoansHeldForSaleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_LoansAndLeasesHeldForInvestmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_AllowanceForCreditLossesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_PremisesAndEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_CoreDepositIntangiblesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_DeferredTaxAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=fibk_TotalAssetAdjustmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | fibk_AssetAdjustmentsAxis=us-gaap_OtherAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |