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Document and Entity Information (USD $)
12 Months Ended
Dec. 31, 2012
Jun. 30, 2012
Jan. 31, 2013
Class A Common Stock
Jan. 31, 2013
Class B Common Stock
Entity Registrant Name FIRST INTERSTATE BANCSYSTEM INC      
Entity Central Index Key 0000860413      
Current Fiscal Year End Date --12-31      
Entity Filer Category Accelerated Filer      
Document Type 10-K      
Document Period End Date Dec. 31, 2012      
Document Fiscal Year Focus 2012      
Document Fiscal Period Focus FY      
Amendment Flag false      
Entity Common Stock, Shares Outstanding     17,691,998 25,639,335
Entity Well-known Seasoned Issuer No      
Entity Voluntary Filers No      
Entity Current Reporting Status Yes      
Entity Public Float   $ 265,747,635