Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Jan. 31, 2012 Common Class A	Jan. 31, 2012 Common Class B
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-K
Document Period End Date	Dec. 31, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding			16,477,353	26,544,097
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float		$ 275,147,011