| Segment Reporting [Table Text Block] |
| (Dollars in thousands) | | Mercantile Bank | | | Eastern Michigan Bank | | | All Other | | | Intercompany Eliminations | | | Consolidated Total | |
| | | | | | | | | | | | | | | | | | | | | |
| Interest income | | $ | 160,309 | | | $ | 12,891 | | | $ | 2,122 | | | $ | (3,200 | ) | | $ | 172,122 | |
| Interest expense | | | 54,944 | | | | 1,142 | | | | 6,017 | | | | (3,140 | ) | | | 58,963 | |
| Net interest income | | | 105,365 | | | | 11,749 | | | | (3,895 | ) | | | (60 | ) | | | 113,159 | |
| Provision for credit losses | | | (3,400 | ) | | | (200 | ) | | | - | | | | - | | | | (3,600 | ) |
| Net interest income after provision for credit losses | | | 108,765 | | | | 11,949 | | | | (3,895 | ) | | | (60 | ) | | | 116,759 | |
| Noninterest income | | | 21,886 | | | | 1,132 | | | | 39,169 | | | | (39,000 | ) | | | 23,187 | |
| Noninterest expense | | | 69,533 | | | | 8,045 | | | | 3,905 | | | | - | | | | 81,483 | |
| Income before federal income tax expense | | | 61,118 | | | | 5,036 | | | | 31,369 | | | | (39,060 | ) | | | 58,463 | |
| Federal income tax expense | | | 10,550 | | | | 1,033 | | | | (1,732 | ) | | | - | | | | 9,851 | |
| Net income | | $ | 50,568 | | | $ | 4,003 | | | $ | 33,101 | | | $ | (39,060 | ) | | $ | 48,612 | |
| | | | | | | | | | | | | | | | | | | | | |
| Goodwill | | $ | 32,171 | | | $ | 24,217 | | | $ | 17,301 | | | $ | - | | | $ | 73,689 | |
| Core deposit intangible, net | | | - | | | | 17,307 | | | | - | | | | - | | | | 17,307 | |
| Total assets | | | 6,240,346 | | | | 553,623 | | | | 1,046,837 | | | | (1,021,316 | ) | | | 6,819,490 | |
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