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| 0000860413-25-000060.txt | 2025-05-07 14:47:01 | |
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| fibk-20250331.htm | 3251089 | 2025-05-07 14:47:01 |
| fibk-20250331.xsd | 68400 | 2025-05-07 14:47:01 |
| fibk-20250331xex311.htm | 10215 | 2025-05-07 14:47:01 |
| fibk-20250331xex312.htm | 10362 | 2025-05-07 14:47:01 |
| fibk-20250331xex32.htm | 4069 | 2025-05-07 14:47:01 |
| fibk-20250331_cal.xml | 126026 | 2025-05-07 14:47:01 |
| fibk-20250331_def.xml | 610759 | 2025-05-07 14:47:01 |
| fibk-20250331_htm.xml | 4081551 | 2025-05-07 14:47:01 |
| fibk-20250331_lab.xml | 840851 | 2025-05-07 14:47:01 |
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| FilingSummary.xml | 43990 | 2025-05-07 14:47:01 |
| Financial_Report.xlsx | 178806 | 2025-05-07 14:47:01 |
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| report.css | 2767 | 2025-05-07 14:47:01 |
| Show.js | 1085 | 2025-05-07 14:47:01 |