Document and Entity Information	3 Months Ended
	Mar. 31, 2018 shares
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-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Class A Common Stock
Entity Common Stock, Shares Outstanding	34,067,912
Class B Common Stock
Entity Common Stock, Shares Outstanding	22,631,503