XML 123 R45.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Other Real Estate Owned (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2017
Repossessed Assets [Abstract] |
Other Real Estate Owned | Information with respect to the Company’s other real estate owned follows:
Year Ended December 31, | 2017 |  | 2016 |  | 2015
Balance at beginning of year | $ | 10.0 |  |  | $ | 6.3 |  |  | $ | 13.6 |
Acquisitions | 1.2 |  |  | 1.1 |  |  | — |
Additions | 5.4 |  |  | 7.6 |  |  | 5.6 |
Valuation adjustments | (0.4 | ) |  | (0.6 | ) |  | (0.2 | )
Dispositions | (6.1 | ) |  | (4.4 | ) |  | (12.7 | )
Balance at end of year | $ | 10.1 |  |  | $ | 10.0 |  |  | $ | 6.3 |

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