REGULATORY CAPITAL (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
REGULATORY CAPITAL
Number of ratios calculated according to the regulations for determining assigned capital category | item	4
Total capital (to risk-weighted assets)
Actual, Amount	$ 344,309	$ 332,726
Required for Adequate Capital, Amount	$ 176,782	$ 165,975
Actual, Ratio	15.60%	16.00%
Required for Adequate Capital, Ratio	8.00%	8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 322,286	$ 309,476
Required for Adequate Capital, Amount	$ 132,587	$ 82,988
Actual, Ratio	14.60%	14.90%
Required for Adequate Capital, Ratio	6.00%	4.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 281,286
Required for Adequate Capital, Amount	$ 99,440
Actual, Ratio	12.70%
Required for Adequate Capital, Ratio	4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 322,286	$ 309,476
Required for Adequate Capital, Amount	$ 128,302	$ 121,014
Actual, Ratio (as a percent)	10.00%	10.20%
Required for Adequate Capital, Ratio	4.00%	4.00%
Cash	$ 112,353	$ 62,485	$ 69,914	$ 61,320
Minimum | Prior To Federal Reserve Corrective Action Rules
Tier 1 capital (to risk-weighted assets)
Required for Adequate Capital, Ratio	4.00%
Minimum | Federal Reserve Corrective Action Rules
Basel III and Dodd Frank Rules
Required minimum conservation buffer percentage	2.50%
Required minimum Tier 1 capital to risk weighted assets (as a percent )	4.50%
Tier 1 capital (to risk-weighted assets)
Required for Adequate Capital, Ratio	6.00%
Tier 1 capital (to average assets)
Required for Adequate Capital, Ratio	4.00%
Parent Company
Tier 1 capital (to average assets)
Cash	$ 8,500
MainSource Bank
Basel III and Dodd Frank Rules
Tier 1 leverage capital required to be well capitalized to average assets (as a percent)	5.00%	5.00%
Total capital (to risk-weighted assets)
Actual, Amount	$ 329,420	$ 321,005
Required for Adequate Capital, Amount	177,083	164,648
To Be Well Capitalized, Amount	$ 231,354	$ 205,810
Actual, Ratio	14.90%	15.60%
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	$ 307,397	$ 297,755
Required for Adequate Capital, Amount	132,812	82,324
To Be Well Capitalized, Amount	$ 177,083	$ 123,486
Actual, Ratio	13.90%	14.50%
Required for Adequate Capital, Ratio	6.00%	4.00%
To Be Well Capitalized, Ratio	8.00%	6.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 307,397
Required for Adequate Capital, Amount	99,609
To Be Well Capitalized, Amount	$ 143,880
Actual, Ratio	13.90%
Required for Adequate Capital, Ratio	4.50%
To Be Well Capitalized, Ratio	6.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 307,397	$ 297,755
Required for Adequate Capital, Amount	127,153	119,880
To Be Well Capitalized, Amount	$ 158,942	$ 149,850
Actual, Ratio (as a percent)	9.70%	9.90%
Required for Adequate Capital, Ratio	4.00%	4.00%
To Be Well Capitalized, Ratio	5.00%	5.00%