| Document and Entity Information | Feb. 26, 2020 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 26, 2020 |
| 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, Suite 1300 |
| Entity Address, City or Town | Chicago |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60631 |
| City Area Code | 708 |
| Local Phone Number | 831-7483 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock |
| Trading Symbol | FMBI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000702325 |
| Amendment Flag | false |
