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Capital Ratios and Shareholders' Equity - Compliance with Regulatory Capital (Details) $ in Thousands | Dec. 31, 2025 USD ($) | Dec. 31, 2024 USD ($)
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Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common equity tier 1 capital to risk weighted assets, Actual, Amount | $ 191,137 | $ 183,348
Common equity tier 1 capital to risk weighted assets, Actual, Ratio | 0.1173 | 0.1221
Common equity tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 114,114 | $ 105,136
Common equity tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 7.00% | 7.00%
Tier 1 capital to risk weighted assets, Actual, Amount | $ 191,137 | $ 183,348
Tier 1 capital to risk weighted assets, Actual, Ratio | 0.1173 | 0.1221
Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 138,568 | $ 127,665
Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 0.0850 | 0.0850
Total capital to risk weighted assets, Actual, Amount | $ 234,871 | $ 226,179
Total capital to risk weighted assets, Actual, Ratio | 0.1441 | 0.1506
Total capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 171,172 | $ 157,703
Total capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 0.1050 | 0.1050
Tier 1 capital to average assets, Actual, Amount | $ 191,137 | $ 183,348
Tier 1 capital to average assets, Actual, Ratio | 0.0884 | 0.0886
Tier 1 capital to average assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 86,476 | $ 82,803
Tier 1 capital to average assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 0.0400 | 0.0400
Isabella Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common equity tier 1 capital to risk weighted assets, Actual, Amount | $ 182,116 | $ 172,589
Common equity tier 1 capital to risk weighted assets, Actual, Ratio | 0.1120 | 0.1153
Common equity tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 113,783 | $ 104,783
Common equity tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 7.00% | 7.00%
Common equity tier 1 capital to risk weighted assets, Minimum Capital Required To Be Considered Well Capitalized, Amount | $ 105,656 | $ 97,299
Common equity tier 1 capital to risk weighted assets, Minimum Capital Required To Be Considered Well Capitalized, Ratio | 0.0650 | 0.0650
Tier 1 capital to risk weighted assets, Actual, Amount | $ 182,116 | $ 172,589
Tier 1 capital to risk weighted assets, Actual, Ratio | 0.1120 | 0.1153
Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 138,165 | $ 127,237
Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 0.0850 | 0.0850
Tier 1 capital to risk weighted assets, Minimum Capital Required To Be Considered Well Capitalized, Amount | $ 130,038 | $ 119,753
Tier 1 capital to risk weighted assets, Minimum Capital Required To Be Considered Well Capitalized, Ratio | 0.0800 | 0.0800
Total capital to risk weighted assets, Actual, Amount | $ 196,336 | $ 185,997
Total capital to risk weighted assets, Actual, Ratio | 0.1208 | 0.1243
Total capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 170,675 | $ 157,175
Total capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 0.1050 | 0.1050
Total capital to risk weighted assets, Minimum Capital Required To Be Considered Well Capitalized, Amount | $ 162,547 | $ 149,691
Total capital to risk weighted assets, Minimum Capital Required To Be Considered Well Capitalized, Ratio | 0.1000 | 0.1000
Tier 1 capital to average assets, Actual, Amount | $ 182,116 | $ 172,589
Tier 1 capital to average assets, Actual, Ratio | 0.0845 | 0.0836
Tier 1 capital to average assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 86,170 | $ 82,602
Tier 1 capital to average assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 0.0400 | 0.0400
Tier 1 capital to average assets, Minimum Capital Required To Be Considered Well Capitalized, Amount | $ 107,713 | $ 103,252
Tier 1 capital to average assets, Minimum Capital Required To Be Considered Well Capitalized, Ratio | 0.0500 | 0.0500

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