XML 66 R45.htm IDEA: XBRL DOCUMENT
v3.22.0.1

Deposits (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2021
Deposits [Abstract] |
Schedule of Deposits, By Type | Deposits are summarized as follows:
December 31, | 2021 |  | 2020
Non-interest bearing demand | $ | 5,568.3 |  |  | $ | 4,633.5 |
Interest bearing: |  |  |
Demand | 4,753.2 |  |  | 4,118.9 |
Savings | 4,981.6 |  |  | 4,405.9 |
Time, $250 and over | 186.7 |  |  | 193.0 |
Time, other | 779.8 |  |  | 865.7 |
Total interest bearing | 10,701.3 |  |  | 9,583.5 |
Total deposits | $ | 16,269.6 |  |  | $ | 14,217.0 |
Schedule of Maturities of Time Deposits | Maturities of time deposits at December 31, 2021 are as follows:
| Time, $250 and Over |  | Total Time
Due within 3 months or less | $ | 38.8 |  |  | $ | 272.7 |
Due after 3 months and within 6 months | 47.2 |  |  | 209.7 |
Due after 6 months and within 12 months | 70.6 |  |  | 293.7 |
Due within 2023 | 18.1 |  |  | 100.6 |
Due within 2024 | 4.5 |  |  | 43.7 |
Due within 2025 | 4.4 |  |  | 28.7 |
Due within 2026 and thereafter | 3.1 |  |  | 17.4 |
Total | $ | 186.7 |  |  | $ | 966.5 |

X
- Definition Schedule of Deposits, By Type [Table Text Block] + References No definition available. + Details
Name: | fibk_ScheduleOfDepositsByTypeTableTextBlock
Namespace Prefix: | fibk_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Schedule of Maturities of Time Deposits [Table Text Block] + References No definition available. + Details
Name: | fibk_ScheduleOfMaturitiesOfTimeDepositsTableTextBlock
Namespace Prefix: | fibk_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

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