v3.25.2
Cover Page - shares
 6 Months Ended
Jun. 30, 2025
Jul. 31, 2025
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2025  
Document Transition Report false  
Entity File Number 001-34653  
Entity Registrant Name FIRST INTERSTATE BANCSYSTEM, INC.  
Entity Incorporation, State or Country Code DE  
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 59101  
City Area Code 406  
Local Phone Number 255-5311  
Title of 12(b) Security Common Stock, $0.00001 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 2025  
Document Fiscal Period Focus Q2  
Entity Common Stock, Shares Outstanding   104,856,752