v3.25.2
Cover
9 Months Ended
Sep. 24, 2025
Sep. 30, 2025
Cover [Abstract]    
Document Type 8-K  
Document Period End Date Sep. 24, 2025  
Entity Registrant Name ISABELLA BANK CORP  
Entity Incorporation, State or Country Code MI  
Entity File Number 000-18415  
Entity Tax Identification Number 38-2830092  
Entity Address, Address Line One 401 North Main Street  
Entity Address, City or Town Mt. Pleasant  
Entity Address, State or Province MI  
Entity Address, Postal Zip Code 48858-1649  
City Area Code 989  
Local Phone Number 772-9471  
Written Communications false  
Soliciting Material false  
Pre-commencement Tender Offer false  
Pre-commencement Issuer Tender Offer false  
Entity Emerging Growth Company false  
Entity Central Index Key 0000842517  
Amendment Flag false  
Current Fiscal Year End Date   --12-31