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Acquisitions - Schedule of Acquired Loans not Deemed to Have Credit Impairment (Details) - Inland Northwest Bank $ in Millions | Aug. 16, 2018 USD ($)
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Business Acquisition [Line Items] |
Contractually required principal and interest payments | $ 894.8
Contractual cash flows not expected to be collected | 26.1
Fair value at acquisition | $ 693.2

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- Definition Loans Acquired in Transfer, Contractually Required Payments Receivable at Acquisition + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Loans Acquired in Transfer, Nonaccretable Difference + References No definition available. + Details
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Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Loans Acquired in Transfer at Acquisition Fair Value + References No definition available. + Details
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Balance Type: | debit
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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
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Period Type: | duration

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |