| Cover | Jan. 17, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 18, 2022 |
| Entity Registrant Name | First Midwest Bancorp, Inc. |
| Entity Central Index Key | 0000702325 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-39320 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Cover [Abstract] | |
| Title of 12(b) Security | Common stock, $0.01 par value |
| Trading Symbol | FMBI |
| Security Exchange Name | NASDAQ |
| Document Information [Line Items] | |
| Security Exchange Name | NASDAQ |
| Trading Symbol | FMBI |
| Title of 12(b) Security | Common stock, $0.01 par value |
| Series A Preferred Stock [Member] | |
| Cover [Abstract] | |
| 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 |
| Document Information [Line Items] | |
| Security Exchange Name | NASDAQ |
| Trading Symbol | FMBIP |
| 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 |
| Series C Preferred Stock [Member] | |
| Cover [Abstract] | |
| 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 |
| Document Information [Line Items] | |
| Security Exchange Name | NASDAQ |
| Trading Symbol | FMBIO |
| 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 |
