REGULATORY CAPITAL (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Apr. 22, 2010
REGULATORY CAPITAL
Number of ratios calculated according to the regulations for determining assigned capital category	3
Total capital (to risk-weighted assets)
Actual, Amount	$ 305,184	$ 313,902
Required for Adequate Capital, Amount	132,169	133,201
Actual, Ratio	18.50%	18.90%
Required for Adequate Capital, Ratio	8.00%	8.00%
To Be Well Capitalized, Ratio			11.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	284,501	293,039
Required for Adequate Capital, Amount	66,085	66,601
Actual, Ratio	17.20%	17.60%
Required for Adequate Capital, Ratio	4.00%	4.00%
Tier 1 capital (to average assets)
Actual, Amount	284,501	293,039
Required for Adequate Capital, Amount	109,279	108,875
Actual, Ratio (as a percent)	10.40%	10.80%
Required for Adequate Capital, Ratio (as a percent)	4.00%	4.00%
To Be Well Capitalized, Ratio (as a percent)			8.00%
Cash	14,000
MainSource Bank
Total capital (to risk-weighted assets)
Actual, Amount	282,963	299,054
Required for Adequate Capital, Amount	130,969	132,001
To Be Well Capitalized, Amount	163,712	181,502
Actual, Ratio	17.30%	18.10%
Required for Adequate Capital, Ratio	8.00%	8.00%
To Be Well Capitalized, Ratio	10.00%	11.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	262,279	278,191
Required for Adequate Capital, Amount	65,485	66,001
To Be Well Capitalized, Amount	98,227
Actual, Ratio	16.00%	16.90%
Required for Adequate Capital, Ratio	4.00%	4.00%
To Be Well Capitalized, Ratio	6.00%
Tier 1 capital (to average assets)
Actual, Amount	262,279	278,191
Required for Adequate Capital, Amount	108,079	107,675
To Be Well Capitalized, Amount	$ 135,074	$ 215,350
Actual, Ratio (as a percent)	9.70%	10.30%
Required for Adequate Capital, Ratio (as a percent)	4.00%	4.00%
To Be Well Capitalized, Ratio (as a percent)	5.00%	8.00%