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Financial Instruments with Off-Balance Sheet Risk (Details) $ in Millions | Sep. 30, 2018 USD ($)
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Entity Information [Line Items] |
Credit extension commitments | $ 2,515.4
Credit extension commitments beyond one year | 942.1
Unused Credit Card Lines |
Entity Information [Line Items] |
Credit extension commitments | 701.8
Standby Letters of Credit |
Entity Information [Line Items] |
Credit extension commitments | $ 45.0

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- 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: | dei_EntityInformationLineItems
Namespace Prefix: | dei_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Credit Extension Commitments + References No definition available. + Details
Name: | fibk_CreditExtensionCommitments
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Credit Extension Commitments, Beyond One Year + References No definition available. + Details
Name: | fibk_CreditExtensionCommitmentsBeyondOneYear
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=fibk_UnusedCreditCardLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |