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Other Real Estate Owned (Tables) | 9 Months Ended
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Sep. 30, 2021
Repossessed Assets [Abstract] |
Other Real Estate Owned Roll Forward | Information with respect to the Company’s other real estate owned is reflected in the following table:
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2021 |  | 2020 |  | 2021 |  | 2020
Beginning balance | $ | 2.0 |  |  | $ | 6.5 |  |  | $ | 2.5 |  |  | $ | 8.5 |
Additions | 0.4 |  |  | 2.0 |  |  | 0.7 |  |  | 3.2 |
Valuation adjustments | — |  |  | (0.1) |  |  | — |  |  | (0.1) |
Dispositions | (0.1) |  |  | (2.7) |  |  | (0.9) |  |  | (5.9) |
Ending balance | $ | 2.3 |  |  | $ | 5.7 |  |  | $ | 2.3 |  |  | $ | 5.7 |

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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_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_ForeclosedAssetsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration