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Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions | Oct. 26, 2020 | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Subsequent Event [Line Items] |  |  |
Financing Receivable, Modifications, Recorded Investment, Non Accrual |  | $ 16.8 | $ 19.4
Class A Common Stock | Subsequent Event |  |  |
Subsequent Event [Line Items] |  |  |
Dividend amount per share (in dollars) | $ 0.38 |  |

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- Definition Financing Receivable, Modifications, Recorded Investment, Non Accrual + References No definition available. + Details
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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=121583591&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_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |