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v3.24.3

Other Real Estate Owned (Tables) | 9 Months Ended
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Sep. 30, 2024
Repossessed Assets [Abstract] |
Other Real Estate Owned Roll Forward | Other real estate owned (“OREO”) is a category of real estate owned by the Company resulting from a default by the borrower. Information with respect to the Company’s OREO is reflected in the following table:
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2024 |  | 2023 |  | 2024 |  | 2023
Beginning balance | $ | 6.7 |  |  | $ | 14.4 |  |  | $ | 16.5 |  |  | $ | 12.7 |
Additions | 0.9 |  |  | — |  |  | 4.6 |  |  | 2.5 |
Valuation adjustments | — |  |  | (0.3) |  |  | (4.2) |  |  | (1.1) |
Dispositions | (3.2) |  |  | (2.5) |  |  | (12.5) |  |  | (2.5) |
Ending balance | $ | 4.4 |  |  | $ | 11.6 |  |  | $ | 4.4 |  |  | $ | 11.6 |

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- Definition Real Estate Acquired Through Foreclosure, Roll Forward [Table Text Block] + References No definition available. + Details
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Namespace Prefix: | fibk_
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- References No definition available. + Details
Name: | us-gaap_ForeclosedAssetsAbstract
Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration