XML 79 R1.htm IDEA: XBRL DOCUMENT v3.8.0.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2017
Jan. 31, 2018
Jun. 30, 2017
Entity Registrant Name FIRST INTERSTATE BANCSYSTEM INC    
Entity Central Index Key 0000860413    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Document Type 10-K    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 1,317,809,107
Class A Common Stock      
Entity Common Stock, Shares Outstanding   33,580,995  
Class B Common Stock      
Entity Common Stock, Shares Outstanding   22,907,947