Regulatory Capital (Regulatory Capital Ratios) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 192,670	$ 189,163
Total capital to risk-weighted assets	11.24%	11.11%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 137,131	$ 136,208
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Tier 1 capital	$ 153,531	$ 149,920
Tier 1 capital to risk-weighted assets	8.96%	8.81%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 102,848	$ 102,156
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Common equity tier 1 capital	$ 143,531	$ 139,920
Common equity tier 1 capital to risk-weighted assets	8.37%	8.22%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 77,136	$ 76,617
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Tier 1 capital	$ 153,531	$ 149,920
Tier 1 leverage capital to average assets	8.44%	8.63%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 72,805	$ 69,466
Tier 1 leverage capital to average assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
First Business Bank
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 145,732	$ 141,388
Total capital to risk-weighted assets	11.43%	11.12%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 102,002	$ 101,754
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Total capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 127,502	$ 127,193
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 132,943	$ 128,852
Tier 1 capital to risk-weighted assets	10.43%	10.13%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 76,501	$ 76,316
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 102,002	$ 101,754
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	$ 132,943	$ 128,852
Common equity tier 1 capital to risk-weighted assets	10.43%	10.13%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 57,376	$ 57,237
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 82,877	$ 110,669
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 capital	$ 132,943	$ 128,852
Tier 1 leverage capital to average assets	10.30%	10.44%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 51,647	$ 49,359
Tier 1 leverage capital to average assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier 1 leverage capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 64,559	$ 61,698
Tier 1 leverage capital to average assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	5.00%	5.00%
First Business Bank - Milwaukee
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 21,170	$ 20,931
Total capital to risk-weighted assets	11.74%	12.03%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 14,425	$ 13,914
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Total capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 18,032	$ 17,392
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 19,350	$ 19,172
Tier 1 capital to risk-weighted assets	10.73%	11.02%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 10,819	$ 10,435
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 14,425	$ 13,914
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	$ 19,350	$ 19,172
Common equity tier 1 capital to risk-weighted assets	10.73%	11.02%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 8,114	$ 7,826
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 11,721	$ 82,675
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 capital	$ 19,350	$ 19,172
Tier 1 leverage capital to average assets	7.38%	7.81%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 10,485	$ 9,821
Tier 1 leverage capital to average assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier 1 leverage capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 13,106	$ 12,276
Tier 1 leverage capital to average assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	5.00%	5.00%
Alterra Bank
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 31,464	$ 30,300
Total capital to risk-weighted assets	12.37%	11.39%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 20,345	$ 21,279
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Total capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 25,432	$ 26,598
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 29,390	$ 28,278
Tier 1 capital to risk-weighted assets	11.56%	10.63%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 15,259	$ 15,959
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 20,345	$ 21,279
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	$ 29,390	$ 28,278
Common equity tier 1 capital to risk-weighted assets	11.56%	10.63%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 11,444	$ 11,969
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 16,530	$ 11,305
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 capital	$ 29,390	$ 28,278
Tier 1 leverage capital to average assets	9.45%	9.89%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 12,440	$ 11,441
Tier 1 leverage capital to average assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier 1 leverage capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 15,551	$ 14,301
Tier 1 leverage capital to average assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	5.00%	5.00%

Common Equity Tier One Capital

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Common Equity Tier One Capital Required for Capital Adequacy to Risk-Weighted Assets

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Common Equity Tier One Capital Required to be Well Capitalized to Risk-Weighted Assets

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Common Equity Tier One Capital to Risk-Weighted Assets

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Common Equity Tier One Capital to be Well Capitalized

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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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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