REGULATORY CAPITAL (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Jun. 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	$ 341,687	$ 332,726
Required for Adequate Capital, Amount	$ 173,374	$ 165,975
Actual, Ratio	15.80%	16.00%
Required for Adequate Capital, Ratio	8.00%	8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 319,441	$ 309,476
Required for Adequate Capital, Amount	$ 130,030	$ 82,988
Actual, Ratio	14.70%	14.90%
Required for Adequate Capital, Ratio	6.00%	4.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 278,441
Required for Adequate Capital, Amount	$ 97,523
Actual, Ratio	12.80%
Required for Adequate Capital, Ratio	4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 319,441	$ 309,476
Required for Adequate Capital, Amount	$ 125,009	$ 121,014
Actual, Ratio (as a percent)	10.20%	10.20%
Required for Adequate Capital, Ratio	4.00%	4.00%
Cash	$ 112,448	$ 62,485	$ 72,771	$ 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	$ 2,500
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	$ 331,826	$ 321,005
Required for Adequate Capital, Amount	173,675	164,648
To Be Well Capitalized, Amount	$ 217,093	$ 205,810
Actual, Ratio	15.30%	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	$ 309,353	$ 297,755
Required for Adequate Capital, Amount	130,256	82,324
To Be Well Capitalized, Amount	$ 173,675	$ 123,486
Actual, Ratio	14.20%	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	$ 309,353
Required for Adequate Capital, Amount	97,692
To Be Well Capitalized, Amount	$ 141,111
Actual, Ratio	14.20%
Required for Adequate Capital, Ratio	4.50%
To Be Well Capitalized, Ratio	6.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 309,353	$ 297,755
Required for Adequate Capital, Amount	123,860	119,880
To Be Well Capitalized, Amount	$ 154,825	$ 149,850
Actual, Ratio (as a percent)	10.00%	9.90%
Required for Adequate Capital, Ratio	4.00%	4.00%
To Be Well Capitalized, Ratio	5.00%	5.00%