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v3.20.2

Other Real Estate Owned (Tables) | 6 Months Ended
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Jun. 30, 2020
Repossessed Assets [Abstract] |
Other Real Estate Owned Roll Forward | Information with respect to the Company’s other real estate owned follows:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2020 |  | 2019 |  | 2020 |  | 2019
Beginning balance | $ | 8.2 |  |  | $ | 21.1 |  |  | $ | 8.5 |  |  | $ | 14.4 |
OREO acquired through acquisition | — |  |  | 2.4 |  |  | — |  |  | 2.4 |
Additions | 0.3 |  |  | 4.9 |  |  | 1.2 |  |  | 13.3 |
Valuation adjustments | — |  |  | (0.1 | ) |  | — |  |  | (0.4 | )
Dispositions | (2.0 | ) |  | (0.7 | ) |  | (3.2 | ) |  | (2.1 | )
Ending balance | $ | 6.5 |  |  | $ | 27.6 |  |  | $ | 6.5 |  |  | $ | 27.6 |

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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_
Data Type: | nonnum: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