| Document And Entity Information | Apr. 20, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | Mercantile Bank Corporation |
| Document, Type | 8-K |
| Document, Period End Date | Apr. 20, 2021 |
| Entity, Incorporation, State or Country Code | MI |
| Entity, File Number | 000-26719 |
| Entity, Tax Identification Number | 38-3360865 |
| Entity, Address, Address Line One | 310 Leonard Street NW |
| Entity, Address, City or Town | Grand Rapids |
| Entity, Address, State or Province | MI |
| Entity, Address, Postal Zip Code | 49504 |
| City Area Code | 616 |
| Local Phone Number | 406-3000 |
| 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 | MBWM |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001042729 |
