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Document and Entity Information
6 Months Ended
Jun. 30, 2014
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-Q
Document Period End Date Jun. 30, 2014
Document Fiscal Year Focus 2014
Document Fiscal Period Focus Q2
Amendment Flag false
Class A Common Stock
 
Entity Common Stock, Shares Outstanding 20,269,171
Class B Common Stock
 
Entity Common Stock, Shares Outstanding 23,985,841