    <SEC-DOCUMENT>0000860413-21-000204-index.html : 20211026
    <SEC-HEADER>0000860413-21-000204.hdr.sgml : 20211026
    <ACCEPTANCE-DATETIME>20211026160921
    ACCESSION NUMBER:		0000860413-21-000204
    CONFORMED SUBMISSION TYPE:	8-K
    PUBLIC DOCUMENT COUNT:		70
    CONFORMED PERIOD OF REPORT:	20211025
    ITEM INFORMATION:		Results of Operations and Financial Condition
    ITEM INFORMATION:		Regulation FD Disclosure
    ITEM INFORMATION:		Other Events
    ITEM INFORMATION:		Financial Statements and Exhibits
    FILED AS OF DATE:		20211026
    DATE AS OF CHANGE:		20211026

    FILER:

    	COMPANY DATA:
    		COMPANY CONFORMED NAME:			FIRST INTERSTATE BANCSYSTEM INC
    		CENTRAL INDEX KEY:			0000860413
    		STANDARD INDUSTRIAL CLASSIFICATION:	STATE COMMERCIAL BANKS [6022]
    		IRS NUMBER:				810331430
    		STATE OF INCORPORATION:			MT
    		FISCAL YEAR END:			1231

    	FILING VALUES:
    		FORM TYPE:		8-K
    		SEC ACT:		1934 Act
    		SEC FILE NUMBER:	001-34653
    		FILM NUMBER:		211348486

    	BUSINESS ADDRESS:
    		STREET 1:		P O BOX 30918
    		STREET 2:		401 NO 31ST STREET
    		CITY:			BILLINGS
    		STATE:			MT
    		ZIP:			59116-0918
    		BUSINESS PHONE:		4062555300

    	MAIL ADDRESS:
    		STREET 1:		P O BOX 30918
    		STREET 2:		401 NO 31ST STREET
    		CITY:			BILLINGS
    		STATE:			MT
    		ZIP:			59116-0918

    	FORMER COMPANY:
    		FORMER CONFORMED NAME:	FIRST INTERSTATE BANCSYSTEM OF MONTANA INC
    		DATE OF NAME CHANGE:	19930615
    </SEC-HEADER>
    <DOCUMENT>
    <TYPE>8-K
    <SEQUENCE>1
    <FILENAME>fibk-20211025.htm
    <DESCRIPTION>8-K
    <TEXT>
    Document 1 - file: fibk-20211025.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-99.1
    <SEQUENCE>2
    <FILENAME>fibk-20210930x8kpr.htm
    <DESCRIPTION>EX-99.1
    <TEXT>
    Document 2 - file: fibk-20210930x8kpr.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-99.2
    <SEQUENCE>3
    <FILENAME>oct2021ip.htm
    <DESCRIPTION>EX-99.2
    <TEXT>
    Document 3 - file: oct2021ip.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.SCH
    <SEQUENCE>4
    <FILENAME>fibk-20211025.xsd
    <DESCRIPTION>XBRL TAXONOMY EXTENSION SCHEMA DOCUMENT
    <TEXT>
    Document 4 - file: fibk-20211025.xsd

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.LAB
    <SEQUENCE>5
    <FILENAME>fibk-20211025_lab.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION LABEL LINKBASE DOCUMENT
    <TEXT>
    Document 5 - file: fibk-20211025_lab.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-101.PRE
    <SEQUENCE>6
    <FILENAME>fibk-20211025_pre.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION PRESENTATION LINKBASE DOCUMENT
    <TEXT>
    Document 6 - file: fibk-20211025_pre.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>7
    <FILENAME>fibslogo2linea04a.jpg
    <DESCRIPTION>FIBK LOGO
    <TEXT>
    Document 7 - file: fibslogo2linea04a.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>8
    <FILENAME>oct2021ip001.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 8 - file: oct2021ip001.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>9
    <FILENAME>oct2021ip002.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 9 - file: oct2021ip002.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>10
    <FILENAME>oct2021ip003.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 10 - file: oct2021ip003.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>11
    <FILENAME>oct2021ip004.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 11 - file: oct2021ip004.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>12
    <FILENAME>oct2021ip005.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 12 - file: oct2021ip005.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>13
    <FILENAME>oct2021ip006.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 13 - file: oct2021ip006.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>14
    <FILENAME>oct2021ip007.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 14 - file: oct2021ip007.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>15
    <FILENAME>oct2021ip008.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 15 - file: oct2021ip008.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>16
    <FILENAME>oct2021ip009.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 16 - file: oct2021ip009.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>17
    <FILENAME>oct2021ip010.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 17 - file: oct2021ip010.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>18
    <FILENAME>oct2021ip011.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 18 - file: oct2021ip011.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>19
    <FILENAME>oct2021ip012.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 19 - file: oct2021ip012.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>20
    <FILENAME>oct2021ip013.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 20 - file: oct2021ip013.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>21
    <FILENAME>oct2021ip014.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 21 - file: oct2021ip014.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>22
    <FILENAME>oct2021ip015.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 22 - file: oct2021ip015.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>23
    <FILENAME>oct2021ip016.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 23 - file: oct2021ip016.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>24
    <FILENAME>oct2021ip017.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 24 - file: oct2021ip017.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>25
    <FILENAME>oct2021ip018.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 25 - file: oct2021ip018.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>26
    <FILENAME>oct2021ip019.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 26 - file: oct2021ip019.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>27
    <FILENAME>oct2021ip020.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 27 - file: oct2021ip020.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>28
    <FILENAME>oct2021ip021.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 28 - file: oct2021ip021.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>29
    <FILENAME>oct2021ip022.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 29 - file: oct2021ip022.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>30
    <FILENAME>oct2021ip023.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 30 - file: oct2021ip023.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>31
    <FILENAME>oct2021ip024.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 31 - file: oct2021ip024.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>32
    <FILENAME>oct2021ip025.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 32 - file: oct2021ip025.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>33
    <FILENAME>oct2021ip026.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 33 - file: oct2021ip026.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>34
    <FILENAME>oct2021ip027.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 34 - file: oct2021ip027.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>35
    <FILENAME>oct2021ip028.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 35 - file: oct2021ip028.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>36
    <FILENAME>oct2021ip029.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 36 - file: oct2021ip029.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>37
    <FILENAME>oct2021ip030.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 37 - file: oct2021ip030.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>38
    <FILENAME>oct2021ip031.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 38 - file: oct2021ip031.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>39
    <FILENAME>oct2021ip032.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 39 - file: oct2021ip032.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>40
    <FILENAME>oct2021ip033.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 40 - file: oct2021ip033.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>41
    <FILENAME>oct2021ip034.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 41 - file: oct2021ip034.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>42
    <FILENAME>oct2021ip035.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 42 - file: oct2021ip035.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>43
    <FILENAME>oct2021ip036.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 43 - file: oct2021ip036.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>44
    <FILENAME>oct2021ip037.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 44 - file: oct2021ip037.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>45
    <FILENAME>oct2021ip038.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 45 - file: oct2021ip038.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>46
    <FILENAME>oct2021ip039.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 46 - file: oct2021ip039.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>47
    <FILENAME>oct2021ip040.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 47 - file: oct2021ip040.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>48
    <FILENAME>oct2021ip041.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 48 - file: oct2021ip041.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>49
    <FILENAME>oct2021ip042.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 49 - file: oct2021ip042.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>50
    <FILENAME>oct2021ip043.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 50 - file: oct2021ip043.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>51
    <FILENAME>oct2021ip044.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 51 - file: oct2021ip044.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>52
    <FILENAME>oct2021ip045.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 52 - file: oct2021ip045.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>53
    <FILENAME>oct2021ip046.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 53 - file: oct2021ip046.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>54
    <FILENAME>oct2021ip047.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 54 - file: oct2021ip047.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>55
    <FILENAME>oct2021ip048.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 55 - file: oct2021ip048.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>56
    <FILENAME>oct2021ip049.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 56 - file: oct2021ip049.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>57
    <FILENAME>oct2021ip050.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 57 - file: oct2021ip050.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>58
    <FILENAME>oct2021ip051.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 58 - file: oct2021ip051.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>59
    <FILENAME>oct2021ip052.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 59 - file: oct2021ip052.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>60
    <FILENAME>oct2021ip053.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 60 - file: oct2021ip053.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>61
    <FILENAME>oct2021ip054.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 61 - file: oct2021ip054.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>62
    <FILENAME>oct2021ip055.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 62 - file: oct2021ip055.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>63
    <FILENAME>fibk-20211025_htm.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 63 - file: fibk-20211025_htm.html

    Document 63 - RAW XML: fibk-20211025_htm.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>64
    <FILENAME>R1.htm
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 64 - file: R1.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EXCEL
    <SEQUENCE>65
    <FILENAME>Financial_Report.xlsx
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 65 - file: Financial_Report.xlsx

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>66
    <FILENAME>Show.js
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 66 - file: Show.js

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>67
    <FILENAME>report.css
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 67 - file: report.css

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>68
    <FILENAME>FilingSummary.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 68 - file: FilingSummary.html

    Document 68 - RAW XML: FilingSummary.xml

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>JSON
    <SEQUENCE>70
    <FILENAME>MetaLinks.json
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 70 - file: MetaLinks.json

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>ZIP
    <SEQUENCE>71
    <FILENAME>0000860413-21-000204-xbrl.zip
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 71 - file: 0000860413-21-000204-xbrl.zip

    </DOCUMENT>
    </SEC-DOCUMENT>