| Cover | Jul. 22, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jul. 22, 2021 |
| 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 |
