    <SEC-DOCUMENT>0000860413-20-000101-index.html : 20200323
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    <ACCEPTANCE-DATETIME>20200320192617
    ACCESSION NUMBER:		0000860413-20-000101
    CONFORMED SUBMISSION TYPE:	8-K
    PUBLIC DOCUMENT COUNT:		21
    CONFORMED PERIOD OF REPORT:	20200320
    ITEM INFORMATION:		Regulation FD Disclosure
    ITEM INFORMATION:		Financial Statements and Exhibits
    FILED AS OF DATE:		20200323
    DATE AS OF CHANGE:		20200320

    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:		20733308

    	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-20200320x8k.htm
    <DESCRIPTION>8-K
    <TEXT>
    Document 1 - file: fibk-20200320x8k.htm

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>EX-99.1
    <SEQUENCE>2
    <FILENAME>hospitalitypresentation.htm
    <DESCRIPTION>EXHIBIT 99.1
    <TEXT>
    Document 2 - file: hospitalitypresentation.htm

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    <DOCUMENT>
    <TYPE>EX-101.SCH
    <SEQUENCE>3
    <FILENAME>fibk-20200320.xsd
    <DESCRIPTION>XBRL TAXONOMY EXTENSION SCHEMA DOCUMENT
    <TEXT>
    Document 3 - file: fibk-20200320.xsd

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    <FILENAME>fibk-20200320_cal.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION CALCULATION LINKBASE DOCUMENT
    <TEXT>
    Document 4 - file: fibk-20200320_cal.xml

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    <SEQUENCE>5
    <FILENAME>fibk-20200320_def.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION DEFINITION LINKBASE DOCUMENT
    <TEXT>
    Document 5 - file: fibk-20200320_def.xml

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    <TYPE>EX-101.LAB
    <SEQUENCE>6
    <FILENAME>fibk-20200320_lab.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION LABEL LINKBASE DOCUMENT
    <TEXT>
    Document 6 - file: fibk-20200320_lab.xml

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    <TYPE>EX-101.PRE
    <SEQUENCE>7
    <FILENAME>fibk-20200320_pre.xml
    <DESCRIPTION>XBRL TAXONOMY EXTENSION PRESENTATION LINKBASE DOCUMENT
    <TEXT>
    Document 7 - file: fibk-20200320_pre.xml

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    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>8
    <FILENAME>hospitalitypresentation001.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 8 - file: hospitalitypresentation001.jpg

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    <DOCUMENT>
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    <SEQUENCE>9
    <FILENAME>hospitalitypresentation002.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 9 - file: hospitalitypresentation002.jpg

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    <DOCUMENT>
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    <SEQUENCE>10
    <FILENAME>hospitalitypresentation003.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 10 - file: hospitalitypresentation003.jpg

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    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>11
    <FILENAME>hospitalitypresentation004.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 11 - file: hospitalitypresentation004.jpg

    </DOCUMENT>
    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>12
    <FILENAME>hospitalitypresentation005.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 12 - file: hospitalitypresentation005.jpg

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    <DOCUMENT>
    <TYPE>GRAPHIC
    <SEQUENCE>13
    <FILENAME>hospitalitypresentation006.jpg
    <DESCRIPTION>GRAPHIC
    <TEXT>
    Document 13 - file: hospitalitypresentation006.jpg

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>14
    <FILENAME>fibk-20200320x8k_htm.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 14 - file: fibk-20200320x8k_htm.html

    Document 14 - RAW XML: fibk-20200320x8k_htm.xml

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    <TYPE>ZIP
    <SEQUENCE>15
    <FILENAME>0000860413-20-000101-xbrl.zip
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 15 - file: 0000860413-20-000101-xbrl.zip

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    <DOCUMENT>
    <TYPE>EXCEL
    <SEQUENCE>16
    <FILENAME>Financial_Report.xlsx
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 16 - file: Financial_Report.xlsx

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>18
    <FILENAME>Show.js
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 18 - file: Show.js

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    <DOCUMENT>
    <TYPE>JSON
    <SEQUENCE>19
    <FILENAME>MetaLinks.json
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 19 - file: MetaLinks.json

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>20
    <FILENAME>FilingSummary.xml
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 20 - file: FilingSummary.html

    Document 20 - RAW XML: FilingSummary.xml

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>21
    <FILENAME>R1.htm
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 21 - file: R1.htm

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    <DOCUMENT>
    <TYPE>XML
    <SEQUENCE>22
    <FILENAME>report.css
    <DESCRIPTION>IDEA: XBRL DOCUMENT
    <TEXT>
    Document 22 - file: report.css

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