XML 11 R1.htm IDEA: XBRL DOCUMENT v3.7.0.1
Document and Entity Information
3 Months Ended
Mar. 31, 2017
shares
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 Mar. 31, 2017
Document Fiscal Year Focus 2017
Document Fiscal Period Focus Q1
Amendment Flag false
Class A Common Stock  
Entity Common Stock, Shares Outstanding 21,926,590
Class B Common Stock  
Entity Common Stock, Shares Outstanding 23,217,418