Note 18 - Regulatory and Capital Matters (Detail) - Summary of Capital Ratios (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Adequately Capitalized Capital (in Dollars)	$ 18,300	$ 19,300
Actual Ratio [Member] | First Midwest Bancorp, Inc. [Member]
Actual Ratio	13.68%	16.27%
Actual Ratio	11.61%	14.20%
Actual Ratio	9.28%	11.21%
Actual Ratio [Member] | First Midwest Bank [Member]
Actual Ratio	14.37%	13.87%
Actual Ratio	13.11%	12.61%
Actual Ratio	10.37%	9.88%
Adequately Capitalized Ratio [Member] | First Midwest Bancorp, Inc. [Member]
Adequately Capitalized Ratio	8.00%	8.00%
Adequately Capitalized Ratio	4.00%	4.00%
Well-Capitalized for FDICIA Ratio	4.00%	4.00%
Adequately Capitalized Ratio	3.00%	3.00%
Adequately Capitalized Ratio [Member] | First Midwest Bank [Member]
Adequately Capitalized Ratio	8.00%	8.00%
Adequately Capitalized Ratio	4.00%	4.00%
Well-Capitalized for FDICIA Ratio	4.00%	4.00%
Adequately Capitalized Ratio	3.00%	3.00%
FDICIA Ratio [Member] | First Midwest Bancorp, Inc. [Member]
Well-Capitalized for FDICIA Ratio	10.00%	10.00%
Adequately Capitalized Ratio	6.00%	6.00%
Well-Capitalized for FDICIA Ratio	6.00%	6.00%
Well-Capitalized for FDICIA Ratio	5.00%	5.00%
FDICIA Ratio [Member] | First Midwest Bank [Member]
Well-Capitalized for FDICIA Ratio	10.00%	10.00%
Adequately Capitalized Ratio	6.00%	6.00%
Well-Capitalized for FDICIA Ratio	6.00%	6.00%
Well-Capitalized for FDICIA Ratio	5.00%	5.00%
Actual Capital [Member] | First Midwest Bancorp, Inc. [Member]
Actual Capital (in Dollars)	853,961	1,027,761
Actual Capital (in Dollars)	724,863	897,410
Actual Capital (in Dollars)	724,863	897,410
Actual Capital [Member] | First Midwest Bank [Member]
Actual Capital (in Dollars)	880,223	856,027
Actual Capital (in Dollars)	803,054	778,008
Actual Capital (in Dollars)	803,054	778,008
Adequately Capitalized Capital [Member] | First Midwest Bancorp, Inc. [Member]
Adequately Capitalized Capital (in Dollars)	499,295	505,420
Adequately Capitalized Capital (in Dollars)	249,648	252,710
Adequately Capitalized Capital (in Dollars)	234,409	240,066
Adequately Capitalized Capital [Member] | First Midwest Bank [Member]
Adequately Capitalized Capital (in Dollars)	489,968	493,622
Adequately Capitalized Capital (in Dollars)	244,984	246,811
Adequately Capitalized Capital (in Dollars)	232,370	236,253
FDICIA Capital [Member] | First Midwest Bancorp, Inc. [Member]
Well-Capitalized for FDICIA Capital (in Dollars)	624,119	631,774
Well-Capitalized for FDICIA Capital (in Dollars)	374,471	379,065
Well-Capitalized for FDICIA Capital (in Dollars)	390,682	400,107
FDICIA Capital [Member] | First Midwest Bank [Member]
Well-Capitalized for FDICIA Capital (in Dollars)	612,459	617,027
Well-Capitalized for FDICIA Capital (in Dollars)	367,476	370,216
Well-Capitalized for FDICIA Capital (in Dollars)	$ 387,284	$ 393,755