Document and Entity Information	9 Months Ended
	Sep. 30, 2016 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	Sep. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q3
Amendment Flag	false
Class A Common Stock
Entity Common Stock, Shares Outstanding	21,459,481
Class B Common Stock
Entity Common Stock, Shares Outstanding	23,420,517