REGULATORY CAPITAL (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Actual and required capital amounts and ratios
Number of ratios calculated according to the regulations for determining assigned capital category | item	4
Total capital (to risk-weighted assets)
Actual, Amount	$ 389,211	$ 356,210
Required for Adequate Capital, Amount	$ 214,229	$ 184,170
Actual, Ratio	14.50%	15.50%
Required for Adequate Capital, Ratio	8.00%	8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 367,727	$ 334,190
Required for Adequate Capital, Amount	$ 160,672	$ 138,127
Actual, Ratio	13.70%	14.50%
Required for Adequate Capital, Ratio	6.00%	6.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 326,853	$ 293,316
Required for Adequate Capital, Amount	$ 120,504	$ 103,595
Actual, Ratio	12.20%	12.70%
Required for Adequate Capital, Ratio	4.50%	4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 367,727	$ 334,190
Required for Adequate Capital, Amount	$ 142,744	$ 130,845
Actual, Ratio (as a percent)	10.30%	10.20%
Required for Adequate Capital, Ratio	4.00%	4.00%
Cash and cash equivalents	$ 88,428	$ 67,578	$ 112,448	$ 62,485
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 Tier 1 capital to risk weighted assets (as a percent )	4.50%
Required minimum conservation buffer percentage	2.50%
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 and cash equivalents	$ 21,000
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	$ 390,722	$ 334,266
Required for Adequate Capital, Amount	214,228	184,168
To Be Well Capitalized, Amount	$ 267,784	$ 230,211
Actual, Ratio	14.60%	14.50%
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	$ 369,238	$ 312,246
Required for Adequate Capital, Amount	160,671	138,126
To Be Well Capitalized, Amount	$ 214,228	$ 184,168
Actual, Ratio	13.80%	13.60%
Required for Adequate Capital, Ratio	6.00%	6.00%
To Be Well Capitalized, Ratio	8.00%	8.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 369,238	$ 312,246
Required for Adequate Capital, Amount	120,503	103,595
To Be Well Capitalized, Amount	$ 174,060	$ 149,637
Actual, Ratio	13.80%	13.60%
Required for Adequate Capital, Ratio	4.50%	4.50%
To Be Well Capitalized, Ratio	6.50%	6.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 369,238	$ 312,246
Required for Adequate Capital, Amount	142,454	130,555
To Be Well Capitalized, Amount	$ 178,067	$ 163,194
Actual, Ratio (as a percent)	10.40%	9.60%
Required for Adequate Capital, Ratio	4.00%	4.00%
To Be Well Capitalized, Ratio	5.00%	5.00%