| Segment Reporting [Table Text Block] |
| (Dollars in thousands) | | Mercantile Bank | | | Eastern Michigan Bank | | | All Other | | | Intercompany Eliminations | | | Consolidated Total | |
| | | | | | | | | | | | | | | | | | | | | |
| Interest income | | $ | 244,702 | | | $ | 19,315 | | | $ | 3,196 | | | $ | (4,822 | ) | | $ | 262,391 | |
| Interest expense | | | 82,268 | | | | 1,771 | | | | 9,023 | | | | (4,731 | ) | | | 88,331 | |
| Net interest income | | | 162,434 | | | | 17,544 | | | | (5,827 | ) | | | (91 | ) | | | 174,060 | |
| Provision for credit losses | | | (2,700 | ) | | | (200 | ) | | | - | | | | - | | | | (2,900 | ) |
| Net interest income after provision for credit losses | | | 165,134 | | | | 17,744 | | | | (5,827 | ) | | | (91 | ) | | | 176,960 | |
| Noninterest income | | | 33,637 | | | | 1,584 | | | | 58,716 | | | | (58,500 | ) | | | 35,437 | |
| Noninterest expense | | | 106,948 | | | | 12,101 | | | | 5,934 | | | | - | | | | 124,983 | |
| Income before federal income tax expense | | | 91,823 | | | | 7,227 | | | | 46,955 | | | | (58,591 | ) | | | 87,414 | |
| Federal income tax expense | | | 15,794 | | | | 1,482 | | | | (2,547 | ) | | | - | | | | 14,729 | |
| Net income | | $ | 76,029 | | | $ | 5,745 | | | $ | 49,502 | | | $ | (58,591 | ) | | $ | 72,685 | |
| | | | | | | | | | | | | | | | | | | | | |
| Goodwill | | $ | 32,171 | | | $ | 24,217 | | | $ | 17,301 | | | $ | - | | | $ | 73,689 | |
| Core deposit intangible, net | | | - | | | | 16,442 | | | | - | | | | - | | | | 16,442 | |
| Total assets | | | 6,529,958 | | | | 587,643 | | | | 1,051,866 | | | | (1,006,599 | ) | | | 7,162,868 | |
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