REGULATORY MATTERS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2014 USD ($)	Jan. 01, 2019	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)
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	$ 407,802			$ 356,210
Required for Adequate Capital, Amount	$ 222,095			$ 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	$ 385,303			$ 334,190
Required for Adequate Capital, Amount	$ 166,571			$ 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	$ 344,212			$ 293,316
Required for Adequate Capital, Amount	$ 124,928			$ 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	$ 385,303			$ 334,190
Required for Adequate Capital, Amount	$ 157,671			$ 130,845
Actual, Ratio (as a percent)	9.80%			10.20%
Required for Adequate Capital, Ratio	4.00%			4.00%
Cash and cash equivalents	$ 86,552	$ 62,485		$ 67,578	$ 61,320
Prior To Federal Reserve Corrective Action Rules
Basel III and Dodd Frank Rules
Required minimum Tier 1 capital to risk weighted assets (as a percent )		4.00%
Federal Reserve Corrective Action Rules
Basel III and Dodd Frank Rules
Capital conservation buffer, percentage	0.625%
Required minimum Tier 1 capital to risk weighted assets (as a percent )	6.00%
Tier 1 capital (to average assets)
Required for Adequate Capital, Ratio	4.00%
Forecast 2019
Basel III and Dodd Frank Rules
Capital conservation buffer, percentage			2.50%
Parent Company
Tier 1 capital (to average assets)
Cash and cash equivalents	$ 10,366	$ 6,322		$ 14,846	$ 11,667
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	$ 407,939			$ 334,266
Required for Adequate Capital, Amount	222,152			184,168
To Be Well Capitalized, Amount	$ 277,690			$ 230,211
Actual, Ratio	14.70%			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	$ 385,440			$ 312,246
Required for Adequate Capital, Amount	166,614			138,126
To Be Well Capitalized, Amount	$ 222,152			$ 184,168
Actual, Ratio	13.90%			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	$ 385,440			$ 312,246
Required for Adequate Capital, Amount	124,960			103,595
To Be Well Capitalized, Amount	$ 180,498			$ 149,637
Actual, Ratio	13.90%			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	$ 385,440			$ 312,246
Required for Adequate Capital, Amount	157,390			130,555
To Be Well Capitalized, Amount	$ 196,737			$ 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%