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Other Real Estate Owned (Tables) | 12 Months Ended
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Dec. 31, 2019
Repossessed Assets [Abstract] |
Other Real Estate Owned | Information with respect to the Company’s other real estate owned follows:
Year Ended December 31, | 2019 |  | 2018 |  | 2017
Balance at beginning of year | $ | 14.4 |  |  | $ | 10.1 |  |  | $ | 10.0 |
Acquisitions | 2.4 |  |  | 0.6 |  |  | 1.2 |
Additions | 14.1 |  |  | 12.1 |  |  | 5.4 |
Capitalized improvements | 0.3 |  |  | — |  |  | — |
Valuation adjustments | (0.9 | ) |  | (0.1 | ) |  | (0.4 | )
Dispositions | (21.8 | ) |  | (8.3 | ) |  | (6.1 | )
Balance at end of year | $ | 8.5 |  |  | $ | 14.4 |  |  | $ | 10.1 |

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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
Period Type: | duration

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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