REGULATORY CAPITAL (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 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	$ 306,978	$ 296,714
Required for Adequate Capital, Amount	136,821	133,549
Actual, Ratio	17.90%	17.80%
Required for Adequate Capital, Ratio	8.00%	8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	285,698	275,707
Required for Adequate Capital, Amount	68,425	66,774
Actual, Ratio	16.70%	16.50%
Required for Adequate Capital, Ratio	4.00%	4.00%
Tier 1 capital (to average assets)
Actual, Amount	285,698	275,707
Required for Adequate Capital, Amount	107,903	106,077
Actual, Ratio (as a percent)	10.60%	10.40%
Required for Adequate Capital, Ratio (as a percent)	4.00%	4.00%
Cash	57,008	65,650	102,312	109,148
Parent Company
Tier 1 capital (to average assets)
Cash	20,000
MainSource Bank
Total capital (to risk-weighted assets)
Actual, Amount	280,624	283,303
Required for Adequate Capital, Amount	135,651	132,052
To Be Well Capitalized, Amount	169,526	165,065
Actual, Ratio	16.60%	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	259,344	262,527
Required for Adequate Capital, Amount	67,825	66,026
To Be Well Capitalized, Amount	101,716	99,039
Actual, Ratio	15.30%	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	259,344	262,527
Required for Adequate Capital, Amount	107,103	106,077
To Be Well Capitalized, Amount	$ 133,860	$ 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%