Cover Page	9 Months Ended
	Sep. 30, 2021 shares
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	001-34653
Entity Registrant Name	FIRST INTERSTATE BANCSYSTEM, INC.
Entity Incorporation, State or Country Code	MT
Entity Tax Identification Number	81-0331430
Entity Address, Address Line One	401 North 31st Street
Entity Address, City or Town	Billings,
Entity Address, State or Province	MT
Entity Address, Postal Zip Code	59116-0918
City Area Code	406
Local Phone Number	255-5390
Title of 12(b) Security	Class A common stock, no par value
Trading Symbol	FIBK
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Amendment Flag	false
Entity Central Index Key	0000860413
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Class A Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding	41,715,627
Class B Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding	20,515,516