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Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Entity Information |  |
Credit extension commitments | $ 2,620.4 | $ 2,179.5
Unused credit card lines | 1,058.5 | 775.0
Unused Credit Card Lines |  |
Entity Information |  |
Credit extension commitments | 726.7 | 635.3
Standby Letters of Credit |  |
Entity Information |  |
Credit extension commitments | $ 46.7 | $ 50.5

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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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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
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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
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Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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