REGULATORY CAPITAL (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) item	Jan. 01, 2019	Dec. 31, 2015 USD ($)	Sep. 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
Basel III and Dodd Frank Rules
Capital conservation buffer, percentage			0.00%
Total capital (to risk-weighted assets)
Actual, Amount	$ 397,269		$ 356,210
Required for Adequate Capital, Amount	$ 215,540		$ 184,170
Actual, Ratio	14.70%		15.50%
Required for Adequate Capital, Ratio	8.00%		8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 375,441		$ 334,190
Required for Adequate Capital, Amount	$ 161,655		$ 138,127
Actual, Ratio	13.90%		14.50%
Required for Adequate Capital, Ratio	6.00%		6.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 334,567		$ 293,316
Required for Adequate Capital, Amount	$ 121,241		$ 103,595
Actual, Ratio	12.40%		12.70%
Required for Adequate Capital, Ratio	4.50%		4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 375,441		$ 334,190
Required for Adequate Capital, Amount	$ 155,726		$ 130,845
Actual, Ratio (as a percent)	9.60%		10.20%
Required for Adequate Capital, Ratio	4.00%		4.00%
Cash and cash equivalents	$ 80,329		$ 67,578	$ 112,353	$ 62,485
Federal Reserve Corrective Action Rules
Basel III and Dodd Frank Rules
Capital conservation buffer, percentage	0.625%
Forecast 2019
Basel III and Dodd Frank Rules
Capital conservation buffer, percentage		2.50%
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	$ 4,000
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	$ 403,466		$ 334,266
Required for Adequate Capital, Amount	215,538		184,168
To Be Well Capitalized, Amount	$ 269,423		$ 230,211
Actual, Ratio	15.00%		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	$ 381,638		$ 312,246
Required for Adequate Capital, Amount	161,654		138,126
To Be Well Capitalized, Amount	$ 215,538		$ 184,168
Actual, Ratio	14.20%		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	$ 381,638		$ 312,246
Required for Adequate Capital, Amount	121,240		103,595
To Be Well Capitalized, Amount	$ 175,125		$ 149,637
Actual, Ratio	14.20%		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	$ 381,638		$ 312,246
Required for Adequate Capital, Amount	155,435		130,555
To Be Well Capitalized, Amount	$ 194,294		$ 163,194
Actual, Ratio (as a percent)	9.80%		9.60%
Required for Adequate Capital, Ratio	4.00%		4.00%
To Be Well Capitalized, Ratio	5.00%		5.00%