Regulatory Capital (Regulatory Capital Ratios) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 181,281	$ 173,263
Total capital to risk-weighted assets	11.11%	12.13%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 130,502	$ 114,253
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Tier 1 capital	$ 143,156	$ 136,008
Tier 1 capital to risk-weighted assets	8.78%	9.52%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 97,877	$ 57,127
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	4.00%
Common equity tier 1 capital	$ 133,156
Common equity tier 1 capital to risk-weighted assets	8.16%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 73,407
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%
Tier 1 capital	$ 143,156	$ 136,008
Tier 1 leverage capital to average assets	8.66%	8.71%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 5,326	$ 62,490
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	$ 135,519	$ 131,411
Total capital to risk-weighted assets	11.23%	12.19%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 96,523	$ 86,272
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	$ 120,654	$ 107,841
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 123,424	$ 118,907
Tier 1 capital to risk-weighted assets	10.23%	11.03%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 72,393	$ 43,136
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	4.00%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 96,523	$ 64,704
Tier 1 capital to risk weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	8.00%	6.00%
Common equity tier 1 capital	$ 123,424
Common equity tier 1 capital to risk-weighted assets	10.23%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 54,294
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 106,033
Common equity tier 1 capital to risk-weighted assets, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	6.50%
Tier 1 capital	$ 123,424	$ 118,907
Tier 1 leverage capital to average assets	10.55%	10.13%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 4,937	$ 46,960
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	$ 6,171	$ 58,700
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	$ 19,404	$ 19,128
Total capital to risk-weighted assets	11.27%	12.47%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 13,779	$ 12,274
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	$ 17,223	$ 15,343
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 17,746	$ 17,641
Tier 1 capital to risk-weighted assets	10.30%	11.50%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 10,334	$ 6,137
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	4.00%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 13,779	$ 9,206
Tier 1 capital to risk weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	8.00%	6.00%
Common equity tier 1 capital	$ 17,746
Common equity tier 1 capital to risk-weighted assets	10.30%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 7,751
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 78,425
Common equity tier 1 capital to risk-weighted assets, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	6.50%
Tier 1 capital	$ 17,746	$ 17,641
Tier 1 leverage capital to average assets	7.46%	7.90%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 710	$ 8,935
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	$ 887	$ 11,169
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	$ 27,979	$ 22,657
Total capital to risk-weighted assets	10.79%	10.90%
Total capital, Minimum Required for Capital Adequacy Purposes	$ 20,748	$ 16,628
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	$ 25,935	$ 20,785
Total capital to risk-weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 capital	$ 26,533	$ 22,320
Tier 1 capital to risk-weighted assets	10.23%	10.74%
Tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 15,561	$ 8,314
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	4.00%
Tier 1 capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	$ 20,748	$ 12,471
Tier 1 capital to risk weighted assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	8.00%	6.00%
Common equity tier 1 capital	$ 26,533
Common equity tier 1 capital to risk-weighted assets	10.23%
Common equity tier 1 capital, Minimum Required for Capital Adequacy Purposes	$ 11,671
Common equity tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	4.50%
Common equity tier 1 capital, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	$ 11,195
Common equity tier 1 capital to risk-weighted assets, Minimum Required to Be Well Capitalized Under Prompt Corrective Action Requirements	6.50%
Tier 1 capital	$ 26,533	$ 22,320
Tier 1 leverage capital to average assets	10.00%	9.01%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	$ 1,061	$ 9,910
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	$ 1,327	$ 12,388
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 under 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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Tier 1 capital divided by risk weighted assets as defined by regulations.

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