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Capital Ratios and Shareholders' Equity - Compliance with Regulatory Capital Requirements (Details) $ in Thousands | Sep. 30, 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 | $ 189,547 | $ 183,348
Common equity Tier 1 capital to risk weighted assets, Actual, Ratio | 0.1237 | 0.1221
Common equity Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 107,266 | $ 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 | $ 189,547 | $ 183,348
Tier 1 capital to risk weighted assets, Actual, Ratio | 0.1237 | 0.1221
Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 130,251 | $ 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 | $ 232,859 | $ 226,179
Total capital to risk weighted assets, Actual, Ratio | 0.1520 | 0.1506
Total capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 160,898 | $ 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 | $ 189,547 | $ 183,348
Tier 1 capital to average assets, Actual, Ratio | 0.0871 | 0.0886
Tier 1 capital to average assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 87,019 | $ 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 | $ 187,258 | $ 172,589
Common equity Tier 1 capital to risk weighted assets, Actual, Ratio | 0.1226 | 0.1153
Common equity Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 106,874 | $ 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 | $ 99,240 | $ 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 | $ 187,258 | $ 172,589
Tier 1 capital to risk weighted assets, Actual, Ratio | 0.1226 | 0.1153
Tier 1 capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 129,776 | $ 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 | $ 122,142 | $ 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 | $ 201,078 | $ 185,997
Total capital to risk weighted assets, Actual, Ratio | 0.1317 | 0.1243
Total capital to risk weighted assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 160,311 | $ 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 | $ 152,677 | $ 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 | $ 187,258 | $ 172,589
Tier 1 capital to average assets, Actual, Ratio | 0.0863 | 0.0836
Tier 1 capital to average assets, Minimum Capital Required Plus Capital Conservation Buffer, Amount | $ 86,788 | $ 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 | $ 108,486 | $ 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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