Document and Entity Information	6 Months Ended
	Jun. 30, 2012
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	Jun. 30, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q2
Amendment Flag	false
Class A Common Stock
Entity Common Stock, Shares Outstanding	16,890,002
Class B Common Stock
Entity Common Stock, Shares Outstanding	26,338,748