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Other Real Estate Owned - Narrative (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Balance at beginning of year |  |
Carrying values of foreclosed residential real estate properties | $ 0.0 |
Consumer mortgage loans collateralized by residential real estate property in the process of foreclosure | $ 1.0 | $ 0.5

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- Definition Foreclosed Residential Real Estate included in Other Real Estate Owned + References No definition available. + Details
Name: | fibk_ForeclosedResidentialRealEstateincludedinOtherRealEstateOwned
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Real Estate Acquired Through Foreclosure [Roll Forward] + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureRollForward
Namespace Prefix: | fibk_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 35 -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-35 + Details
Name: | us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant