Parent Directory| Name | Size | Last Modified |
|---|---|---|
 0000860413-24-000016-index-headers.html | 2024-02-29 14:46:18 | |
| 0000860413-24-000016-index.html | 2024-02-29 14:46:18 | |
| 0000860413-24-000016.txt | 2024-02-29 14:46:18 | |
 0000860413-24-000016-xbrl.zip | 971232 | 2024-02-29 14:46:18 |
| fibk-20231231.htm | 5703589 | 2024-02-29 14:46:18 |
| fibk-20231231.xsd | 135121 | 2024-02-29 14:46:18 |
| fibk-20231231xex1018_direc.htm | 11510 | 2024-02-29 14:46:18 |
| fibk-20231231xex141_codeof.htm | 25316 | 2024-02-29 14:46:18 |
| fibk-20231231xex231_consen.htm | 3370 | 2024-02-29 14:46:18 |
| fibk-20231231xex311_certxc.htm | 9996 | 2024-02-29 14:46:18 |
| fibk-20231231xex312_certxc.htm | 9960 | 2024-02-29 14:46:18 |
| fibk-20231231xex32_section.htm | 6183 | 2024-02-29 14:46:18 |
| fibk-20231231xex41_descxse.htm | 11958 | 2024-02-29 14:46:18 |
| fibk-20231231xex97_clawback.htm | 37045 | 2024-02-29 14:46:18 |
| fibk-20231231_cal.xml | 218258 | 2024-02-29 14:46:18 |
| fibk-20231231_def.xml | 1016146 | 2024-02-29 14:46:18 |
 fibk-20231231_g1.jpg | 141355 | 2024-02-29 14:46:18 |
| fibk-20231231_htm.xml | 5960593 | 2024-02-29 14:46:18 |
| fibk-20231231_lab.xml | 1742022 | 2024-02-29 14:46:18 |
| fibk-20231231_pre.xml | 1346307 | 2024-02-29 14:46:18 |
| fibk-20240123xex1023_robbi.htm | 111741 | 2024-02-29 14:46:18 |
| fibk-20240226xex1012rsuagmt.htm | 72912 | 2024-02-29 14:46:18 |
| fibk-20240226xex1013rsuagm.htm | 97424 | 2024-02-29 14:46:18 |
| fibk-20240226xex1014_prsur.htm | 94595 | 2024-02-29 14:46:18 |
| fibk-20240226xex1015_prsut.htm | 94631 | 2024-02-29 14:46:18 |
| fibk-20240226xex1016_prsur.htm | 121226 | 2024-02-29 14:46:18 |
| fibk-20240226xex1017_prsut.htm | 122320 | 2024-02-29 14:46:18 |
| FilingSummary.xml | 72880 | 2024-02-29 14:46:18 |
| Financial_Report.xlsx | 316360 | 2024-02-29 14:46:18 |
 image_0.jpg | 54496 | 2024-02-29 14:46:18 |
| MetaLinks.json | 1816381 | 2024-02-29 14:46:18 |
| R1.htm | 63001 | 2024-02-29 14:46:18 |
| R10.htm | 20279 | 2024-02-29 14:46:18 |
| R100.htm | 28040 | 2024-02-29 14:46:18 |
| R101.htm | 30992 | 2024-02-29 14:46:18 |
| R102.htm | 17427 | 2024-02-29 14:46:18 |
| R103.htm | 35329 | 2024-02-29 14:46:18 |
| R104.htm | 21542 | 2024-02-29 14:46:18 |
| R105.htm | 62100 | 2024-02-29 14:46:18 |
| R106.htm | 4741 | 2024-02-29 14:46:18 |
| R107.htm | 48794 | 2024-02-29 14:46:18 |
| R108.htm | 74422 | 2024-02-29 14:46:18 |
| R109.htm | 17189 | 2024-02-29 14:46:18 |
| R11.htm | 193641 | 2024-02-29 14:46:18 |
| R110.htm | 67563 | 2024-02-29 14:46:18 |
| R111.htm | 29189 | 2024-02-29 14:46:18 |
| R112.htm | 57405 | 2024-02-29 14:46:18 |
| R113.htm | 61866 | 2024-02-29 14:46:18 |
| R114.htm | 64559 | 2024-02-29 14:46:18 |
| R115.htm | 154172 | 2024-02-29 14:46:18 |
| R116.htm | 69393 | 2024-02-29 14:46:18 |
| R117.htm | 13661 | 2024-02-29 14:46:18 |
| R118.htm | 83490 | 2024-02-29 14:46:18 |
| R119.htm | 22860 | 2024-02-29 14:46:18 |
| R12.htm | 82107 | 2024-02-29 14:46:18 |
| R120.htm | 39288 | 2024-02-29 14:46:18 |
| R121.htm | 17314 | 2024-02-29 14:46:18 |
| R13.htm | 69273 | 2024-02-29 14:46:18 |
| R14.htm | 45300 | 2024-02-29 14:46:18 |
| R15.htm | 305895 | 2024-02-29 14:46:18 |
| R16.htm | 30382 | 2024-02-29 14:46:18 |
| R17.htm | 898741 | 2024-02-29 14:46:18 |
| R18.htm | 15168 | 2024-02-29 14:46:18 |
| R19.htm | 14211 | 2024-02-29 14:46:18 |
| R2.htm | 7795 | 2024-02-29 14:46:18 |
| R20.htm | 18946 | 2024-02-29 14:46:18 |
| R21.htm | 172627 | 2024-02-29 14:46:18 |
| R22.htm | 31471 | 2024-02-29 14:46:18 |
| R23.htm | 28990 | 2024-02-29 14:46:18 |
| R24.htm | 43915 | 2024-02-29 14:46:18 |
| R25.htm | 68140 | 2024-02-29 14:46:18 |
| R26.htm | 17174 | 2024-02-29 14:46:18 |
| R27.htm | 21197 | 2024-02-29 14:46:18 |
| R28.htm | 87568 | 2024-02-29 14:46:18 |
| R29.htm | 24666 | 2024-02-29 14:46:18 |
| R3.htm | 4499 | 2024-02-29 14:46:18 |
| R30.htm | 18726 | 2024-02-29 14:46:18 |
| R31.htm | 78323 | 2024-02-29 14:46:18 |
| R32.htm | 20163 | 2024-02-29 14:46:18 |
| R33.htm | 6180 | 2024-02-29 14:46:18 |
| R34.htm | 49463 | 2024-02-29 14:46:18 |
| R35.htm | 88262 | 2024-02-29 14:46:18 |
| R36.htm | 258251 | 2024-02-29 14:46:18 |
| R37.htm | 9853 | 2024-02-29 14:46:18 |
| R38.htm | 36549 | 2024-02-29 14:46:18 |
| R39.htm | 5974 | 2024-02-29 14:46:18 |
| R4.htm | 19870 | 2024-02-29 14:46:18 |
| R40.htm | 144402 | 2024-02-29 14:46:18 |
| R41.htm | 58581 | 2024-02-29 14:46:18 |
| R42.htm | 51206 | 2024-02-29 14:46:18 |
| R43.htm | 303212 | 2024-02-29 14:46:18 |
| R44.htm | 30437 | 2024-02-29 14:46:18 |
| R45.htm | 415578 | 2024-02-29 14:46:18 |
| R46.htm | 13854 | 2024-02-29 14:46:18 |
| R47.htm | 11211 | 2024-02-29 14:46:18 |
| R48.htm | 17006 | 2024-02-29 14:46:18 |
| R49.htm | 24326 | 2024-02-29 14:46:18 |
| R5.htm | 139466 | 2024-02-29 14:46:18 |
| R50.htm | 27578 | 2024-02-29 14:46:18 |
| R51.htm | 30271 | 2024-02-29 14:46:18 |
| R52.htm | 82985 | 2024-02-29 14:46:18 |
| R53.htm | 19764 | 2024-02-29 14:46:18 |
| R54.htm | 80372 | 2024-02-29 14:46:18 |
| R55.htm | 12419 | 2024-02-29 14:46:18 |
| R56.htm | 51069 | 2024-02-29 14:46:18 |
| R57.htm | 87306 | 2024-02-29 14:46:18 |
| R58.htm | 245477 | 2024-02-29 14:46:18 |
| R59.htm | 45263 | 2024-02-29 14:46:18 |
| R6.htm | 5280 | 2024-02-29 14:46:18 |
| R60.htm | 47448 | 2024-02-29 14:46:18 |
| R61.htm | 81970 | 2024-02-29 14:46:18 |
| R62.htm | 4876 | 2024-02-29 14:46:18 |
| R63.htm | 28054 | 2024-02-29 14:46:18 |
| R64.htm | 19178 | 2024-02-29 14:46:18 |
| R65.htm | 34044 | 2024-02-29 14:46:18 |
| R66.htm | 17445 | 2024-02-29 14:46:18 |
| R67.htm | 9171 | 2024-02-29 14:46:18 |
| R68.htm | 21079 | 2024-02-29 14:46:18 |
| R69.htm | 26087 | 2024-02-29 14:46:18 |
| R7.htm | 116923 | 2024-02-29 14:46:18 |
| R70.htm | 24109 | 2024-02-29 14:46:18 |
| R71.htm | 153237 | 2024-02-29 14:46:18 |
| R72.htm | 69571 | 2024-02-29 14:46:18 |
| R73.htm | 63324 | 2024-02-29 14:46:18 |
| R74.htm | 15354 | 2024-02-29 14:46:18 |
| R75.htm | 111282 | 2024-02-29 14:46:18 |
| R76.htm | 61568 | 2024-02-29 14:46:18 |
| R77.htm | 100228 | 2024-02-29 14:46:18 |
| R78.htm | 98690 | 2024-02-29 14:46:18 |
| R79.htm | 668389 | 2024-02-29 14:46:18 |
| R8.htm | 48280 | 2024-02-29 14:46:18 |
| R80.htm | 391028 | 2024-02-29 14:46:18 |
| R81.htm | 18820 | 2024-02-29 14:46:18 |
| R82.htm | 72676 | 2024-02-29 14:46:18 |
| R83.htm | 6510 | 2024-02-29 14:46:18 |
| R84.htm | 204483 | 2024-02-29 14:46:18 |
| R85.htm | 19899 | 2024-02-29 14:46:18 |
| R86.htm | 13702 | 2024-02-29 14:46:18 |
| R87.htm | 17443 | 2024-02-29 14:46:18 |
| R88.htm | 35232 | 2024-02-29 14:46:18 |
| R89.htm | 41764 | 2024-02-29 14:46:18 |
| R9.htm | 73044 | 2024-02-29 14:46:18 |
| R90.htm | 123900 | 2024-02-29 14:46:18 |
| R91.htm | 7534 | 2024-02-29 14:46:18 |
| R92.htm | 21573 | 2024-02-29 14:46:18 |
| R93.htm | 22353 | 2024-02-29 14:46:18 |
| R94.htm | 58616 | 2024-02-29 14:46:18 |
| R95.htm | 120705 | 2024-02-29 14:46:18 |
| R96.htm | 164974 | 2024-02-29 14:46:18 |
| R97.htm | 60834 | 2024-02-29 14:46:18 |
| R98.htm | 32680 | 2024-02-29 14:46:18 |
| R99.htm | 84654 | 2024-02-29 14:46:18 |
| report.css | 2768 | 2024-02-29 14:46:18 |
| Show.js | 1086 | 2024-02-29 14:46:18 |