REGULATORY CAPITAL (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) item	Dec. 31, 2019	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
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	$ 442,560		$ 407,802
Required for Adequate Capital, Amount	$ 263,158		$ 222,095
Actual, Ratio	13.50%		14.70%
Required for Adequate Capital, Ratio	8.00%		8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 420,017		$ 385,303
Required for Adequate Capital, Amount	$ 197,369		$ 166,571
Actual, Ratio	12.80%		13.90%
Required for Adequate Capital, Ratio	6.00%		6.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 372,091		$ 344,212
Required for Adequate Capital, Amount	$ 148,026		$ 124,928
Actual, Ratio	11.30%		12.40%
Required for Adequate Capital, Ratio	4.50%		4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 420,017		$ 385,303
Required for Adequate Capital, Amount	$ 177,717		$ 157,671
Actual, Ratio (as a percent)	9.50%		9.80%
Required for Adequate Capital, Ratio	4.00%		4.00%
Cash and cash equivalents	$ 70,277		$ 86,552	$ 80,329	$ 67,578
Federal Reserve Corrective Action Rules
Basel III and Dodd Frank Rules
Capital conservation buffer, percentage	1.25%
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	$ 13,000
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	$ 446,375		$ 407,939
Required for Adequate Capital, Amount	263,215		222,152
To Be Well Capitalized, Amount	$ 329,019		$ 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	$ 423,832		$ 385,440
Required for Adequate Capital, Amount	197,411		166,614
To Be Well Capitalized, Amount	$ 263,215		$ 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	$ 423,832		$ 385,440
Required for Adequate Capital, Amount	148,058		124,960
To Be Well Capitalized, Amount	$ 213,862		$ 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	$ 423,832		$ 385,440
Required for Adequate Capital, Amount	177,436		157,390
To Be Well Capitalized, Amount	$ 221,795		$ 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%