Document And Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 03, 2018
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		103,054,177
Amendment Flag	false
Entity Central Index Key	0000702325
Entity Filer Category	Large Accelerated Filer
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2