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Document and Entity Information
12 Months Ended
Dec. 31, 2019
USD ($)
Jan. 31, 2020
shares
Jun. 30, 2019
USD ($)
Apr. 08, 2019
USD ($)
office
$ / shares
shares
Dec. 31, 2018
USD ($)
Entity Information [Line Items]          
Assets $ 14,644,200,000       $ 13,300,200,000
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets 10.50%       9.88%
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets 8.50%       7.88%
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets 7.00%       6.38%
Document Type 10-K        
Document Annual Report true        
Document Period End Date Dec. 31, 2019        
Document Transition Report false        
Entity File Number 001-34653        
Entity Registrant Name FIRST INTERSTATE BANCSYSTEM, INC.        
Entity Central Index Key 0000860413        
Current Fiscal Year End Date --12-31        
Document Fiscal Year Focus 2019        
Document Fiscal Period Focus FY        
Amendment Flag false        
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        
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 Well-known Seasoned Issuer No        
Entity Voluntary Filers No        
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        
Entity Public Float     $ 1,791,530,989    
Documents Incorporated by Reference
The registrant intends to file a definitive Proxy Statement for the Annual Meeting of Shareholders scheduled to be held May 5, 2020. The information required by Part III of this Form 10-K is incorporated by reference to such Proxy Statement.
       
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets 6.50%       6.50%
Common Equity Tier One Risk Based Capital To Risk Weighted Assets 12.62%       11.40%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets 8.00%       8.00%
Tier One Risk Based Capital to Risk Weighted Assets 13.41%       12.26%
Capital Required to be Well Capitalized to Risk Weighted Assets 10.00%       10.00%
Capital to Risk Weighted Assets 14.10%       12.99%
Well capitalized, ratio (percent) 5.00%       5.00%
Tier One Leverage Capital to Average Assets 10.13%       9.47%
Deposits $ 11,663,500,000       $ 10,680,700,000
Loans and Leases Receivable, Allowance $ 9,031,600,000       $ 8,503,700,000
Class A Common Stock          
Entity Information [Line Items]          
Entity Common Stock, Shares Outstanding | shares   43,173,724      
Class B Common Stock          
Entity Information [Line Items]          
Entity Common Stock, Shares Outstanding | shares   22,110,620      
Community 1st Bank [Member]          
Entity Information [Line Items]          
Number of banking offices | office       3  
Community 1st Bank [Member] | Class A Common Stock          
Entity Information [Line Items]          
Business Acquisition, Share Price | $ / shares       $ 40.64  
Business Acquisition Conversion Share Rate | shares       0.3784  
Idaho Independent Bank [Member]          
Entity Information [Line Items]          
Number of banking offices | office       11  
Class A common stock       $ 157,300,000  
Idaho Independent Bank [Member] | Class A Common Stock          
Entity Information [Line Items]          
Business Acquisition, Share Price | $ / shares       $ 40.64  
Business Acquisition Conversion Share Rate | shares       0.50