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Other Real Estate Owned (Tables) | 3 Months Ended
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Mar. 31, 2018
Repossessed Assets [Abstract] |
Other Real Estate Owned Roll Forward | Information with respect to the Company's other real estate owned follows:
| Three Months Ended March 31,
| 2018 |  | 2017
Beginning balance | $ | 10.1 |  |  | $ | 10.0 |
Additions | 1.1 |  |  | 0.4 |
Valuation adjustments | — |  |  | (0.1 | )
Dispositions | (0.2 | ) |  | (0.9 | )
Ending balance | $ | 11.0 |  |  | $ | 9.4 |

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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