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Other Real Estate Owned (Tables) | 12 Months Ended
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Dec. 31, 2023
Repossessed Assets [Abstract] |
Other Real Estate Owned | Information with respect to the Company’s other real estate owned follows:
Year Ended December 31, | 2023 |  | 2022 |  | 2021
Balance at beginning of year | $ | 12.7 |  |  | $ | 2.0 |  |  | $ | 2.5 |
Acquired through acquisitions | — |  |  | 15.8 |  |  | — |
Additions | 8.3 |  |  | 0.4 |  |  | 0.9 |
Valuation adjustments | (1.1) |  |  | (2.8) |  |  | — |
Dispositions | (3.4) |  |  | (2.7) |  |  | (1.4) |
Balance at end of year | $ | 16.5 |  |  | $ | 12.7 |  |  | $ | 2.0 |

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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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Balance Type: | na
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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration