REGULATORY MATTERS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2015	Jan. 01, 2019	Dec. 31, 2016 USD ($)
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	$ 462,862			$ 407,802
Required for Adequate Capital, Amount	$ 266,667			$ 222,095
Actual, Ratio	13.90%			14.70%
Required for Adequate Capital, Ratio	8.00%			8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 431,920			$ 385,303
Required for Adequate Capital, Amount	$ 200,000			$ 166,571
Actual, Ratio	13.00%			13.90%
Required for Adequate Capital, Ratio	6.00%			6.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 383,757			$ 344,212
Required for Adequate Capital, Amount	$ 150,000			$ 124,928
Actual, Ratio	11.50%			12.40%
Required for Adequate Capital, Ratio	4.50%			4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 431,920			$ 385,303
Required for Adequate Capital, Amount	$ 178,488			$ 157,671
Actual, Ratio (as a percent)	9.70%			9.80%
Required for Adequate Capital, Ratio	4.00%			4.00%
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	1.25%
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%
MainSource Bank
Total capital (to risk-weighted assets)
Actual, Amount	$ 452,411			$ 407,939
Required for Adequate Capital, Amount	266,925			222,152
To Be Well Capitalized, Amount	$ 333,656			$ 277,690
Actual, Ratio	13.60%			14.70%
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	$ 429,868			$ 385,440
Required for Adequate Capital, Amount	200,194			166,614
To Be Well Capitalized, Amount	$ 266,925			$ 222,152
Actual, Ratio	12.90%			13.90%
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	$ 429,868			$ 385,440
Required for Adequate Capital, Amount	150,145			124,960
To Be Well Capitalized, Amount	$ 216,877			$ 180,498
Actual, Ratio	12.90%			13.90%
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	$ 429,868			$ 385,440
Required for Adequate Capital, Amount	178,488			157,390
To Be Well Capitalized, Amount	$ 223,110			$ 196,737
Actual, Ratio (as a percent)	9.60%			9.80%
Required for Adequate Capital, Ratio	4.00%			4.00%
To Be Well Capitalized, Ratio	5.00%			5.00%