v3.26.3
Note 15 - Business Segment Information - Reconciliation of Assets from Segment to Consolidated (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Interest income $ 90,269 $ 85,643 $ 262,391 $ 247,940  
Interest expense 29,369 33,641 88,331 97,911  
Net interest income 60,900 52,002 174,060 150,029  
Provision for credit losses 700 200 (2,900) 3,900  
Net interest income after provision for credit losses 60,200 51,802 176,960 146,129  
Noninterest income 12,303 10,388 35,437 30,552  
Noninterest expense 43,553 34,750 124,983 99,233  
Income before federal income tax expense 28,950 27,440 87,414 77,448  
Federal income tax expense 4,878 3,682 14,729 11,535  
Net income 24,072 $ 23,758 72,685 $ 65,913  
Goodwill 73,689   73,689   $ 72,656
Core deposit intangible, net 16,442   16,442   20,388
Total assets 7,162,868   7,162,868   $ 6,835,219
Core Deposits [Member]          
Core deposit intangible, net 16,442   16,442    
Operating Segments [Member] | Mercantile Bank Segment [Member]          
Interest income     244,702    
Interest expense     82,268    
Net interest income     162,434    
Provision for credit losses     (2,700)    
Net interest income after provision for credit losses     165,134    
Noninterest income     33,637    
Noninterest expense     106,948    
Income before federal income tax expense     91,823    
Federal income tax expense     15,794    
Net income     76,029    
Goodwill 32,171   32,171    
Total assets 6,529,958   6,529,958    
Operating Segments [Member] | Mercantile Bank Segment [Member] | Core Deposits [Member]          
Core deposit intangible, net 0   0    
Operating Segments [Member] | Eastern Michigan Bank Segment [Member]          
Interest income     19,315    
Interest expense     1,771    
Net interest income     17,544    
Provision for credit losses     (200)    
Net interest income after provision for credit losses     17,744    
Noninterest income     1,584    
Noninterest expense     12,101    
Income before federal income tax expense     7,227    
Federal income tax expense     1,482    
Net income     5,745    
Goodwill 24,217   24,217    
Total assets 587,643   587,643    
Operating Segments [Member] | Eastern Michigan Bank Segment [Member] | Core Deposits [Member]          
Core deposit intangible, net 16,442   16,442    
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member]          
Interest income     3,196    
Interest expense     9,023    
Net interest income     (5,827)    
Provision for credit losses     0    
Net interest income after provision for credit losses     (5,827)    
Noninterest income     58,716    
Noninterest expense     5,934    
Income before federal income tax expense     46,955    
Federal income tax expense     (2,547)    
Net income     49,502    
Goodwill 17,301   17,301    
Total assets 1,051,866   1,051,866    
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member] | Core Deposits [Member]          
Core deposit intangible, net 0   0    
Consolidation, Eliminations [Member]          
Interest income     (4,822)    
Interest expense     (4,731)    
Net interest income     (91)    
Provision for credit losses     0    
Net interest income after provision for credit losses     (91)    
Noninterest income     (58,500)    
Noninterest expense     0    
Income before federal income tax expense     (58,591)    
Federal income tax expense     0    
Net income     (58,591)    
Goodwill 0   0    
Total assets (1,006,599)   (1,006,599)    
Consolidation, Eliminations [Member] | Core Deposits [Member]          
Core deposit intangible, net $ 0   $ 0