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Subsequent Events Subsequent Events (Details) - USD ($) | Jan. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Subsequent Event [Line Items] |  |  |
Financing Receivable, CARES Act |  | $ 296,300,000 |
Financing Receivable, CARES Act Applications |  | 3,150 |
Commercial Portfolio Segment [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Loans held for investment |  | $ 2,153,900,000 | $ 1,673,700,000
Class A Common Stock | Subsequent Event |  |  |
Subsequent Event [Line Items] |  |  |
Dividends (in dollars per share) | $ 0.41 |  |

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- Definition Financing Receivable, CARES Act + References No definition available. + Details
Name: | fibk_FinancingReceivableCARESAct
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Financing Receivable, CARES Act Applications + References No definition available. + Details
Name: | fibk_FinancingReceivableCARESActApplications
Namespace Prefix: | fibk_
Data Type: | xbrli:positiveIntegerItemType
Balance Type: | na
Period Type: | instant

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- Definition Gross carrying amount of all categories of loans and leases held in the portfolio, excluding loans held for sale. + References No definition available. + Details
Name: | fibk_LoansAndLeasesReceivableHeldForInvestment
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The per share amount of a dividend declared, but not paid, as of the financial reporting date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |