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Goodwill and Core Deposit Intangibles - Intangible Assets Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items] |  |
Finite-Lived Intangible Asset, Useful Life | 10 years |
Core Deposits Intangibles |  |
Finite-Lived Intangible Assets [Line Items] |  |
Reductions due to sale of accounts | $ 0.0 | $ (0.3)

X
- Definition Finite-Lived Intangible Assets, Increase (Decrease) From Sale Of Accounts + References No definition available. + Details
Name: | fibk_FiniteLivedIntangibleAssetsIncreaseDecreaseFromSaleOfAccounts
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- 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: | us-gaap_FiniteLivedIntangibleAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CoreDepositsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |