v3.26.1
Note 15 - Business Segment Information - Reconciliation of Assets from Segment to Consolidated (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Interest income $ 86,696 $ 81,958 $ 172,122 $ 162,297  
Interest expense 29,438 32,479 58,963 64,270  
Net interest income 57,258 49,479 113,159 98,027  
Provision for credit losses (1,800) 1,600 (3,600) 3,700  
Net interest income after provision for credit losses 59,058 47,879 116,759 94,327  
Noninterest income 11,498 11,462 23,187 20,164  
Noninterest expense 39,375 33,379 81,483 64,483  
Income before federal income tax expense 31,181 25,962 58,463 50,008  
Federal income tax expense 5,254 3,344 9,851 7,853  
Net income 25,927 $ 22,618 48,612 $ 42,155  
Goodwill 73,689   73,689   $ 72,656
Core deposit intangible, net 17,307   17,307   20,388
Total assets 6,819,490   6,819,490   $ 6,835,219
Core Deposits [Member]          
Core deposit intangible, net 17,307   17,307    
Operating Segments [Member] | Mercantile Bank Segment [Member]          
Interest income     160,309    
Interest expense     54,944    
Net interest income     105,365    
Provision for credit losses     (3,400)    
Net interest income after provision for credit losses     108,765    
Noninterest income     21,886    
Noninterest expense     69,533    
Income before federal income tax expense     61,118    
Federal income tax expense     10,550    
Net income     50,568    
Goodwill 32,171   32,171    
Total assets 6,240,346   6,240,346    
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     12,891    
Interest expense     1,142    
Net interest income     11,749    
Provision for credit losses     (200)    
Net interest income after provision for credit losses     11,949    
Noninterest income     1,132    
Noninterest expense     8,045    
Income before federal income tax expense     5,036    
Federal income tax expense     1,033    
Net income     4,003    
Goodwill 24,217   24,217    
Total assets 553,623   553,623    
Operating Segments [Member] | Eastern Michigan Bank Segment [Member] | Core Deposits [Member]          
Core deposit intangible, net 17,307   17,307    
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member]          
Interest income     2,122    
Interest expense     6,017    
Net interest income     (3,895)    
Provision for credit losses     0    
Net interest income after provision for credit losses     (3,895)    
Noninterest income     39,169    
Noninterest expense     3,905    
Income before federal income tax expense     31,369    
Federal income tax expense     (1,732)    
Net income     33,101    
Goodwill 17,301   17,301    
Total assets 1,046,837   1,046,837    
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member] | Core Deposits [Member]          
Core deposit intangible, net 0   0    
Consolidation, Eliminations [Member]          
Interest income     (3,200)    
Interest expense     (3,140)    
Net interest income     (60)    
Provision for credit losses     0    
Net interest income after provision for credit losses     (60)    
Noninterest income     (39,000)    
Noninterest expense     0    
Income before federal income tax expense     (39,060)    
Federal income tax expense     0    
Net income     (39,060)    
Goodwill 0   0    
Total assets (1,021,316)   (1,021,316)    
Consolidation, Eliminations [Member] | Core Deposits [Member]          
Core deposit intangible, net $ 0   $ 0