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Regulatory Capital (Regulatory Capital Ratios) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 242,869	$ 239,029
Total capital to risk-weighted assets	11.74%	12.01%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 165,436	$ 159,185
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 217,135	$ 208,930
Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	10.50%	10.50%
Tier 1 capital	$ 195,318	$ 194,456
Tier 1 capital to risk-weighted assets	9.45%	9.77%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 124,077	$ 119,388
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Tier One Risk Based Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 175,776	$ 169,134
Tier One Risk Based Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	8.50%	8.50%
Common equity tier 1 capital	$ 185,267	$ 184,409
Common equity tier 1 capital to risk-weighted assets	8.96%	9.27%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 93,058	$ 89,541
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Common Equity Tier One Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 144,756	$ 139,286
Common Equity Tier One Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	7.00%	7.00%
Tier 1 leverage capital	$ 195,318	$ 194,456
Tier 1 leverage capital to average assets	9.33%	9.27%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 83,754	$ 83,950
Tier 1 leverage capital to average assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier One Leverage Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 83,754	$ 83,950
Tier One Leverage Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	4.00%	4.00%
First Business Bank
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 234,506	$ 233,181
Total capital to risk-weighted assets	11.41%	11.79%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 164,397	$ 158,177
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 215,771	$ 207,607
Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	10.50%	10.50%
Total capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 205,496	$ 197,721
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 210,672	$ 212,315
Tier 1 capital to risk-weighted assets	10.25%	10.74%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 123,298	$ 118,633
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Tier One Risk Based Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 174,672	$ 168,063
Tier One Risk Based Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	8.50%	8.50%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 164,397	$ 158,177
Tier 1 capital to risk weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	8.00%	8.00%
Common equity tier 1 capital	$ 210,672	$ 212,315
Common equity tier 1 capital to risk-weighted assets	10.25%	10.74%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 92,473	$ 88,974
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Common Equity Tier One Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 143,847	$ 138,405
Common Equity Tier One Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	7.00%	7.00%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 133,572	$ 128,519
Common equity tier 1 capital to risk-weighted assets, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	6.50%	6.50%
Tier 1 leverage capital	$ 210,672	$ 212,315
Tier 1 leverage capital to average assets	10.09%	10.18%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 83,525	$ 83,414
Tier 1 leverage capital to average assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier One Leverage Capital Required for Capital Adequacy Plus Capital Conservation Buffer	$ 83,525	$ 83,414
Tier One Leverage Capital Required for Capital Adequacy Plus Conservation Buffer to Risk Weighted Assets	4.00%	4.00%
Tier 1 leverage capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 104,406	$ 104,268
Tier 1 leverage capital to average assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	5.00%	5.00%

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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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