Document And Entity Information	9 Months Ended
	Sep. 30, 2012	Nov. 09, 2012
Document and Entity Information [Abstract]
Entity Registrant Name	FIRST MIDWEST BANCORP INC
Document Type	10-Q
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		74,843,350
Amendment Flag	false
Entity Central Index Key	0000702325
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	No
Document Period End Date	Sep. 30, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q3