Regulatory Capital (Regulatory Capital Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	$ 177,594us-gaap_Capital	$ 173,263us-gaap_Capital
Total capital to risk-weighted assets	11.40%us-gaap_CapitalToRiskWeightedAssets	12.13%us-gaap_CapitalToRiskWeightedAssets
Total capital, Minimum Required for Capital Adequacy Purposes	124,677	114,253
Total capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Tier 1 capital	139,974us-gaap_TierOneRiskBasedCapital	136,008us-gaap_TierOneRiskBasedCapital
Tier 1 Capital to Risk-Weighted Assets	8.98%	9.52%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	93,508	57,127
Tier 1 capital to risk-weighted assets, Minimum Required for Capital Adequacy Purposes	6.00%	4.00%
Common Equity Tier One Capital	129,974fbiz_CommonEquityTierOneCapital
Common Equity Tier One Capital to Risk-Weighted Assets	8.34%
Common Equity Tier One Capital Required for Capital Adequacy	70,131
Common Equity Tier One Capital Required for Capital Adequacy to Risk-Weighted Assets	4.50%
Tier 1 capital	139,974us-gaap_TierOneLeverageCapital	136,008us-gaap_TierOneLeverageCapital
Tier 1 leverage capital to average assets	8.42%	8.71%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	5,199	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	133,667	131,411
Total capital to risk-weighted assets	11.47%	12.19%
Total capital, Minimum Required for Capital Adequacy Purposes	93,216	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	116,544	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	121,166	118,907
Tier 1 Capital to Risk-Weighted Assets	10.40%	11.03%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	69,912	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	93,235	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 One Capital	121,166
Common Equity Tier One Capital to Risk-Weighted Assets	10.40%
Common Equity Tier One Capital Required for Capital Adequacy	52,434
Common Equity Tier One Capital Required for Capital Adequacy to Risk-Weighted Assets	4.50%
Common Equity Tier One Capital to be Well Capitalized	101,318
Common Equity Tier One Capital Required to be Well Capitalized to Risk-Weighted Assets	6.50%
Tier 1 capital	121,166	118,907
Tier 1 leverage capital to average assets	10.33%	10.13%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	4,847	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	6,061	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	19,215	19,128
Total capital to risk-weighted assets	12.17%	12.47%
Total capital, Minimum Required for Capital Adequacy Purposes	12,626	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	15,783	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	17,721	17,641
Tier 1 Capital to Risk-Weighted Assets	11.23%	11.50%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	9,470	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	12,626	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 One Capital	17,721
Common Equity Tier One Capital to Risk-Weighted Assets	11.23%
Common Equity Tier One Capital Required for Capital Adequacy	7,102
Common Equity Tier One Capital Required for Capital Adequacy to Risk-Weighted Assets	4.50%
Common Equity Tier One Capital to be Well Capitalized	75,753
Common Equity Tier One Capital Required to be Well Capitalized to Risk-Weighted Assets	6.50%
Tier 1 capital	17,721	17,641
Tier 1 leverage capital to average assets	7.56%	7.90%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	709	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	886	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 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations
Total capital	26,346	22,657
Total capital to risk-weighted assets	11.05%	10.90%
Total capital, Minimum Required for Capital Adequacy Purposes	19,072	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	23,841	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	25,647	22,320
Tier 1 Capital to Risk-Weighted Assets	10.76%	10.74%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	14,304	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	19,073	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 One Capital	25,647
Common Equity Tier One Capital to Risk-Weighted Assets	10.76%
Common Equity Tier One Capital Required for Capital Adequacy	10,728
Common Equity Tier One Capital Required for Capital Adequacy to Risk-Weighted Assets	4.50%
Common Equity Tier One Capital to be Well Capitalized	10,259
Common Equity Tier One Capital Required to be Well Capitalized to Risk-Weighted Assets	6.50%
Tier 1 capital	25,647	22,320
Tier 1 leverage capital to average assets	9.68%	9.01%
Tier 1 leverage capital, Minimum Required for Capital Adequacy Purposes	1,026	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	$ 1,270	$ 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 be Well Capitalized

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Common Equity Tier One Capital to Risk-Weighted Assets

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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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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 minimum Tier One Capital Ratio (Tier one 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 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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