XML 60 R44.htm IDEA: XBRL DOCUMENT
v3.20.4

Other Real Estate Owned (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2020
Repossessed Assets [Abstract] |
Other Real Estate Owned | Information with respect to the Company’s other real estate owned follows:
Year Ended December 31, | 2020 |  | 2019 |  | 2018
Balance at beginning of year | $ | 8.5 |  |  | $ | 14.4 |  |  | $ | 10.1 |
OREO acquired through acquisitions | — |  |  | 2.4 |  |  | 0.6 |
Additions | 3.3 |  |  | 14.1 |  |  | 12.1 |
Capitalized improvements | — |  |  | 0.3 |  |  | — |
Valuation adjustments | (0.1) |  |  | (0.9) |  |  | (0.1) |
Dispositions | (9.2) |  |  | (21.8) |  |  | (8.3) |
Balance at end of year | $ | 2.5 |  |  | $ | 8.5 |  |  | $ | 14.4 |

X
- Definition Real Estate Acquired Through Foreclosure, Roll Forward [Table Text Block] + References No definition available. + Details
Name: | fibk_RealEstateAcquiredThroughForeclosureRollForwardTableTextBlock
Namespace Prefix: | fibk_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

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