XML 47 R38.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Comprehensive Text Block List (Tables) | 9 Months Ended
--- | ---
Sep. 30, 2022
Text Block [Abstract] |
financial instruments with off-balance sheet risk | The following table presents our financial instruments with off-balance sheet risk, as well as the activity in the allowance for off-balance sheet credit losses related to those financial instruments:
|  | Three Months Ended September 30, |  | Nine Months Ended September 30,
|  | 2022 |  | 2021 |  | 2022 |  | 2021
Beginning balance |  | $ | 6.2 |  |  | $ | 3.4 |  |  | $ | 3.8 |  |  | $ | 3.7 |
Provision for (reversal of) credit loss expense |  | 3.5 |  |  | (0.2) |  |  | 5.9 |  |  | (0.5) |
Ending balance of allowance for off-balance sheet credit losses |  | $ | 9.7 |  |  | $ | 3.2 |  |  | $ | 9.7 |  |  | $ | 3.2 |
|  | September 30, 2022 |  | December 31, 2021
Unused credit card lines |  | $ | 814.9 |  |  | $ | 681.6 |
Commitments to extend credit |  | 5,011.0 |  |  | 2,539.8 |
Standby letter of credit |  | 92.4 |  |  | 57.5 |

X
- Definition financial instruments with off-balance sheet risk + References No definition available. + Details
Name: | fibk_FinancialInstrumentsWithOffBalanceSheetRiskTableTextBlock
Namespace Prefix: | fibk_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

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