Document and Entity Information (USD $)	9 Months Ended
	Sep. 30, 2011	Jun. 30, 2011
Entity Registrant Name	FIRST INTERSTATE BANCSYSTEM INC
Entity Central Index Key	0000860413
Document Type	10-Q
Document Period End Date	Sep. 30, 2011
Amendment Flag	false
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float		$ 275,147,011
Class A Common Stock
Entity Common Stock, Shares Outstanding	16,336,108
Class B Common Stock
Entity Common Stock, Shares Outstanding	26,643,624