Cover - shares	3 Months Ended
	Mar. 31, 2020	May 05, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	0-10967
Entity Registrant Name	First Midwest Bancorp, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-3161078
Entity Address, Address Line One	8750 West Bryn Mawr Avenue
Entity Address, Address Line Two	Suite 1300
Entity Address, City or Town	Chicago
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60631-3655
City Area Code	708
Local Phone Number	831-7483
Title of 12(b) Security	Common stock, $0.01 par value
Trading Symbol	FMBI
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		114,259,086
Amendment Flag	false
Entity Central Index Key	0000702325
Document Period End Date	Mar. 31, 2020
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1