Regulatory Capital (Regulatory Capital Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Compliance with Regulatory Capital Requirements under Banking Regulations
Total Capital	$ 124,856	$ 118,895
Total Capital to Risk-Weighted Assets	13.31%	13.11%
Total Capital, Minimum Required for Capital Adequacy Purposes	75,083	72,559
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes	8.00%	8.00%
Tier 1 Capital	75,393	71,723
Tier 1 Capital to Risk-Weighted Assets	8.04%	7.91%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	37,541	36,279
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier 1 Capital	75,393	71,723
Tier 1 Capital to Average Assets	6.55%	6.22%
Tier 1 Leverage Capital, Minimum Required for Capital Adequacy Purposes	46,074	46,152
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	114,287	108,860
Total Capital to Risk-Weighted Assets	13.65%	13.39%
Total Capital, Minimum Required for Capital Adequacy Purposes	66,983	65,058
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	83,872	81,322
Total Capital to Risk-Weighted Assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 Capital	103,790	98,666
Tier 1 Capital to Risk-Weighted Assets	12.40%	12.13%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	33,492	32,529
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier 1 Capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	50,323	48,793
Tier 1 Capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	6.00%	6.00%
Tier 1 Capital	103,790	98,666
Tier 1 Capital to Average Assets	10.90%	9.98%
Tier 1 Leverage Capital, Minimum Required for Capital Adequacy Purposes	38,084	39,556
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	47,605	49,445
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	15,315	15,074
Total Capital to Risk-Weighted Assets	15.19%	16.11%
Total Capital, Minimum Required for Capital Adequacy Purposes	8,065	7,484
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	10,082	9,355
Total Capital to Risk-Weighted Assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	10.00%	10.00%
Tier 1 Capital	14,051	13,898
Tier 1 Capital to Risk-Weighted Assets	13.94%	14.86%
Tier 1 Capital, Minimum Required for Capital Adequacy Purposes	4,033	3,742
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes	4.00%	4.00%
Tier 1 Capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	6,049	5,613
Tier 1 Capital, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	6.00%	6.00%
Tier 1 Capital	14,051	13,898
Tier 1 Capital to Average Assets	6.75%	7.95%
Tier 1 Leverage Capital, Minimum Required for Capital Adequacy Purposes	8,325	6,993
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	$ 10,406	$ 8,741
Tier 1 Leverage Capital to Average Assets, Minimum Required to be Well Capitalized Under Prompt Corrective Action Requirements	5.00%	5.00%