Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 24, 2021	Jun. 30, 2020
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	001-39320
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
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 1,491,452,869
Entity Common Stock, Shares Outstanding		114,666,891
Documents Incorporated by Reference	Portions of the Registrant's proxy statement for the 2021 Annual Meeting of Stockholders are incorporated by reference into Part III.
Entity Central Index Key	0000702325
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Common stocks
Document Information [Line Items]
Title of 12(b) Security	Common stock, $0.01 par value
Trading Symbol	FMBI
Security Exchange Name	NASDAQ
Series A Preferred Stock
Document Information [Line Items]
Title of 12(b) Security	Depositary shares, each representing a 1/40th interest in a share of 7.000% Fixed Rate Non-Cumulative Perpetual Preferred Stock, Series A
Trading Symbol	FMBIP
Security Exchange Name	NASDAQ
Series C Preferred Stock
Document Information [Line Items]
Title of 12(b) Security	Depositary shares, each representing a 1/40th interest in a share of 7.000% Fixed Rate Non-Cumulative Perpetual Preferred Stock, Series C
Trading Symbol	FMBIO
Security Exchange Name	NASDAQ