REGULATORY CAPITAL (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 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	$ 310,322	$ 313,902
Required for Adequate Capital, Amount	132,235	133,201
Actual, Ratio	18.80%	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	289,665	293,039
Required for Adequate Capital, Amount	66,118	66,601
Actual, Ratio	17.50%	17.60%
Required for Adequate Capital, Ratio	4.00%	4.00%
Tier 1 capital (to average assets)
Actual, Amount	289,665	293,039
Required for Adequate Capital, Amount	106,383	108,875
Actual, Ratio (as a percent)	10.90%	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	13
MainSource Bank
Total capital (to risk-weighted assets)
Actual, Amount	290,097	299,054
Required for Adequate Capital, Amount	131,035	132,001
To Be Well Capitalized, Amount	163,712
To Comply with Regulatory Agreement		181,502
Actual, Ratio	17.70%	18.10%
Required for Adequate Capital, Ratio	8.00%	8.00%
To Be Well Capitalized, Ratio	10.00%
To Comply with Regulatory Agreement Ratio (as a percent)		11.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	269,442	278,191
Required for Adequate Capital, Amount	65,518	66,001
To Be Well Capitalized, Amount	98,227
Actual, Ratio	16.50%	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	269,442	278,191
Required for Adequate Capital, Amount	105,183	107,675
To Be Well Capitalized, Amount	135,074
To Comply with Regulatory Agreement		$ 215,350
Actual, Ratio (as a percent)	10.20%	10.30%
Required for Adequate Capital, Ratio (as a percent)	4.00%	4.00%
To Be Well Capitalized, Ratio (as a percent)	5.00%
To Comply with Regulatory Agreement (as a percent)		8.00%