Regulatory Capital (Regulatory Capital Ratios) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 200,175	$ 189,163
Total capital to risk-weighted assets	11.44%	11.11%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 139,979	$ 136,208
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Tier 1 capital	$ 157,876	$ 149,920
Tier 1 capital to risk-weighted assets	9.02%	8.81%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 104,984	$ 102,156
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	6.00%
Common equity tier 1 capital	$ 147,876	$ 139,920
Common equity tier 1 capital to risk-weighted assets	8.45%	8.22%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 78,738	$ 76,617
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%	4.50%
Tier 1 leverage capital	$ 157,876	$ 149,920
Tier 1 leverage capital to average assets	8.75%	8.63%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 72,204	$ 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	$ 144,931	$ 141,388
Total capital to risk-weighted assets	11.42%	11.12%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 101,529	$ 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	$ 126,912	$ 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	$ 131,964	$ 128,852
Tier 1 capital to risk-weighted assets	10.40%	10.13%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 76,147	$ 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	$ 101,529	$ 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	$ 131,964	$ 128,852
Common equity tier 1 capital to risk-weighted assets	10.40%	10.13%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 57,110	$ 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	$ 82,493	$ 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 leverage capital	$ 131,964	$ 128,852
Tier 1 leverage capital to average assets	10.34%	10.44%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 51,065	$ 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	$ 63,831	$ 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	$ 23,437	$ 20,931
Total capital to risk-weighted assets	11.12%	12.03%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 16,868	$ 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	$ 21,085	$ 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	$ 21,357	$ 19,172
Tier 1 capital to risk-weighted assets	10.13%	11.02%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 12,651	$ 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	$ 16,868	$ 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	$ 21,357	$ 19,172
Common equity tier 1 capital to risk-weighted assets	10.13%	11.02%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 9,488	$ 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	$ 13,705	$ 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 leverage capital	$ 21,357	$ 19,172
Tier 1 leverage capital to average assets	7.62%	7.81%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 11,205	$ 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	$ 14,006	$ 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	$ 30,705	$ 30,300
Total capital to risk-weighted assets	11.57%	11.39%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 21,225	$ 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	$ 26,531	$ 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	$ 27,368	$ 28,278
Tier 1 capital to risk-weighted assets	10.32%	10.63%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 15,918	$ 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	$ 21,225	$ 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	$ 27,368	$ 28,278
Common equity tier 1 capital to risk-weighted assets	10.32%	10.63%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 11,939	$ 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	$ 17,245	$ 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 leverage capital	$ 27,368	$ 28,278
Tier 1 leverage capital to average assets	9.60%	9.89%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 11,400	$ 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	$ 14,250	$ 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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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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