Document and Entity Information	9 Months Ended
	Sep. 30, 2018 shares
Entity Registrant Name	FIRST INTERSTATE BANCSYSTEM INC
Trading Symbol	FIBK
Entity Central Index Key	0000860413
Document Fiscal Year Focus	2018
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Fiscal Period Focus	Q3
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Entity Emerging Growth Company	false
Entity Small Business	false
Class A Common Stock
Entity Common Stock, Shares Outstanding	38,107,555
Class B Common Stock
Entity Common Stock, Shares Outstanding	22,501,994