REGULATORY CAPITAL (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
REGULATORY CAPITAL
Number of ratios calculated according to the regulations for determining assigned capital category	3
Total capital (to risk-weighted assets)
Actual, Amount	$ 299,513	$ 296,714
Required for Adequate Capital, Amount	132,669	133,549
Actual, Ratio	18.10%	17.80%
Required for Adequate Capital, Ratio	8.00%	8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	278,833	275,707
Required for Adequate Capital, Amount	66,334	66,774
Actual, Ratio	16.80%	16.50%
Required for Adequate Capital, Ratio	4.00%	4.00%
Tier 1 capital (to average assets)
Actual, Amount	278,833	275,707
Required for Adequate Capital, Amount	106,953	106,077
Actual, Ratio (as a percent)	10.40%	10.40%
Required for Adequate Capital, Ratio (as a percent)	4.00%	4.00%
Cash	35,897	65,650	112,159	109,148
Parent Company
Tier 1 capital (to average assets)
Cash	14,000
MainSource Bank
Total capital (to risk-weighted assets)
Actual, Amount	278,743	283,303
Required for Adequate Capital, Amount	131,469	132,052
To Be Well Capitalized, Amount	164,336	165,065
Actual, Ratio	17.00%	17.20%
Required for Adequate Capital, Ratio	8.00%	8.00%
To Be Well Capitalized, Ratio	10.00%	10.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	258,063	262,527
Required for Adequate Capital, Amount	65,734	66,026
To Be Well Capitalized, Amount	98,602	99,039
Actual, Ratio	15.70%	15.90%
Required for Adequate Capital, Ratio	4.00%	4.00%
To Be Well Capitalized, Ratio	6.00%	6.00%
Tier 1 capital (to average assets)
Actual, Amount	258,063	262,527
Required for Adequate Capital, Amount	106,153	106,077
To Be Well Capitalized, Amount	$ 132,691	$ 132,596
Actual, Ratio (as a percent)	9.70%	9.90%
Required for Adequate Capital, Ratio (as a percent)	4.00%	4.00%
To Be Well Capitalized, Ratio (as a percent)	5.00%	5.00%