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Other Real Estate Owned (Tables) | 12 Months Ended
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Dec. 31, 2018
Repossessed Assets [Abstract] |
Other Real Estate Owned | Information with respect to the Company’s other real estate owned follows:
Year Ended December 31, | 2018 |  | 2017 |  | 2016
Balance at beginning of year | $ | 10.1 |  |  | $ | 10.0 |  |  | $ | 6.3 |
Acquisitions | 0.6 |  |  | 1.2 |  |  | 1.1 |
Additions | 12.1 |  |  | 5.4 |  |  | 7.6 |
Valuation adjustments | (0.1 | ) |  | (0.4 | ) |  | (0.6 | )
Dispositions | (8.3 | ) |  | (6.1 | ) |  | (4.4 | )
Balance at end of year | $ | 14.4 |  |  | $ | 10.1 |  |  | $ | 10.0 |

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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
Period Type: | duration

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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