Parent Directory| Name | Size | Last Modified |
|---|---|---|
 0000860413-21-000150-index-headers.html | 2021-05-07 13:12:26 | |
| 0000860413-21-000150-index.html | 2021-05-07 13:12:26 | |
| 0000860413-21-000150.txt | 2021-05-07 13:12:26 | |
 0000860413-21-000150-xbrl.zip | 475788 | 2021-05-07 13:12:26 |
| fibk-20210331.htm | 3775950 | 2021-05-07 13:12:26 |
| fibk-20210331.xsd | 52134 | 2021-05-07 13:12:26 |
| fibk-20210331xex101.htm | 18125 | 2021-05-07 13:12:26 |
| fibk-20210331xex102.htm | 63249 | 2021-05-07 13:12:26 |
| fibk-20210331xex103.htm | 55328 | 2021-05-07 13:12:26 |
| fibk-20210331xex311.htm | 10306 | 2021-05-07 13:12:26 |
| fibk-20210331xex312.htm | 10456 | 2021-05-07 13:12:26 |
| fibk-20210331xex32.htm | 4161 | 2021-05-07 13:12:26 |
| fibk-20210331_cal.xml | 164337 | 2021-05-07 13:12:26 |
| fibk-20210331_def.xml | 482893 | 2021-05-07 13:12:26 |
| fibk-20210331_htm.xml | 4367807 | 2021-05-07 13:12:26 |
| fibk-20210331_lab.xml | 899404 | 2021-05-07 13:12:26 |
| fibk-20210331_pre.xml | 651611 | 2021-05-07 13:12:26 |
| FilingSummary.xml | 36780 | 2021-05-07 13:12:26 |
| Financial_Report.xlsx | 129656 | 2021-05-07 13:12:26 |
| MetaLinks.json | 899865 | 2021-05-07 13:12:26 |
| R1.htm | 54515 | 2021-05-07 13:12:26 |
| R10.htm | 166768 | 2021-05-07 13:12:26 |
| R11.htm | 624163 | 2021-05-07 13:12:26 |
| R12.htm | 14384 | 2021-05-07 13:12:26 |
| R13.htm | 120187 | 2021-05-07 13:12:26 |
| R14.htm | 10863 | 2021-05-07 13:12:26 |
| R15.htm | 17736 | 2021-05-07 13:12:26 |
| R16.htm | 86754 | 2021-05-07 13:12:26 |
| R17.htm | 7029 | 2021-05-07 13:12:26 |
| R18.htm | 20643 | 2021-05-07 13:12:26 |
| R19.htm | 42175 | 2021-05-07 13:12:26 |
| R2.htm | 122594 | 2021-05-07 13:12:26 |
| R20.htm | 204564 | 2021-05-07 13:12:26 |
| R21.htm | 21326 | 2021-05-07 13:12:26 |
| R22.htm | 5585 | 2021-05-07 13:12:26 |
| R23.htm | 23531 | 2021-05-07 13:12:26 |
| R24.htm | 152515 | 2021-05-07 13:12:26 |
| R25.htm | 568721 | 2021-05-07 13:12:26 |
| R26.htm | 12969 | 2021-05-07 13:12:26 |
| R27.htm | 114627 | 2021-05-07 13:12:26 |
| R28.htm | 16572 | 2021-05-07 13:12:26 |
| R29.htm | 84447 | 2021-05-07 13:12:26 |
| R3.htm | 13222 | 2021-05-07 13:12:26 |
| R30.htm | 36367 | 2021-05-07 13:12:26 |
| R31.htm | 192569 | 2021-05-07 13:12:26 |
| R32.htm | 17112 | 2021-05-07 13:12:26 |
| R33.htm | 58133 | 2021-05-07 13:12:26 |
| R34.htm | 37657 | 2021-05-07 13:12:26 |
| R35.htm | 59059 | 2021-05-07 13:12:26 |
| R36.htm | 61132 | 2021-05-07 13:12:26 |
| R37.htm | 178916 | 2021-05-07 13:12:26 |
| R38.htm | 105892 | 2021-05-07 13:12:26 |
| R39.htm | 126316 | 2021-05-07 13:12:26 |
| R4.htm | 122487 | 2021-05-07 13:12:26 |
| R40.htm | 25519 | 2021-05-07 13:12:26 |
| R41.htm | 50819 | 2021-05-07 13:12:26 |
| R42.htm | 137410 | 2021-05-07 13:12:26 |
| R43.htm | 291963 | 2021-05-07 13:12:26 |
| R44.htm | 14502 | 2021-05-07 13:12:26 |
| R45.htm | 45075 | 2021-05-07 13:12:26 |
| R46.htm | 87209 | 2021-05-07 13:12:26 |
| R47.htm | 16911 | 2021-05-07 13:12:26 |
| R48.htm | 32849 | 2021-05-07 13:12:26 |
| R49.htm | 42304 | 2021-05-07 13:12:26 |
| R5.htm | 39205 | 2021-05-07 13:12:26 |
| R50.htm | 86359 | 2021-05-07 13:12:26 |
| R51.htm | 9925 | 2021-05-07 13:12:26 |
| R52.htm | 16948 | 2021-05-07 13:12:26 |
| R53.htm | 43476 | 2021-05-07 13:12:26 |
| R54.htm | 17990 | 2021-05-07 13:12:26 |
| R55.htm | 149024 | 2021-05-07 13:12:26 |
| R56.htm | 35710 | 2021-05-07 13:12:26 |
| R57.htm | 82314 | 2021-05-07 13:12:26 |
| R58.htm | 12488 | 2021-05-07 13:12:26 |
| R59.htm | 9355 | 2021-05-07 13:12:26 |
| R6.htm | 63464 | 2021-05-07 13:12:26 |
| R7.htm | 38663 | 2021-05-07 13:12:26 |
| R8.htm | 118387 | 2021-05-07 13:12:26 |
| R9.htm | 7176 | 2021-05-07 13:12:26 |
| report.css | 2767 | 2021-05-07 13:12:26 |
| Show.js | 1085 | 2021-05-07 13:12:26 |