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Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Entity Information |  |
Credit extension commitments | $ 2,179.5 | $ 1,600.0
Unused credit card lines | 775.0 | 424.5
Unused Credit Card Lines |  |
Entity Information |  |
Credit extension commitments | 635.3 | 544.9
Standby Letters of Credit |  |
Entity Information |  |
Credit extension commitments | $ 50.5 | $ 50.6

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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: |