PARENT ONLY CONDENSED FINANCIAL STATEMENTS - Parent Only Condensed Statement of Income and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expenses
Interest expense									$ 15,678	$ 10,726	$ 8,385
Salaries and benefits									72,614	64,327	57,741
Income before income tax									62,374	50,460	45,775
Income tax (benefit)									12,936	12,137	10,233
Net income attributable to common shareholders	$ 16,595	$ 11,116	$ 9,654	$ 12,073	$ 11,744	$ 11,696	$ 6,117	$ 8,766	49,438	38,323	35,542
Comprehensive income									50,562	28,778	34,453
MainSource Financial Group, Inc.
Income
Dividends from subsidiaries									48,221	43,100	28,900
Fees from subsidiaries									14,353	13,674	13,232
Other income									94	13	19
Total income									62,668	56,787	42,151
Expenses
Interest expense									2,926	1,876	1,264
Salaries and benefits									14,315	13,101	9,873
Professional fees									1,382	1,364	1,238
Other expenses									11,342	9,685	9,233
Total expenses									29,965	26,026	21,608
Income before income taxes and equity in undistributed income of subsidiaries									32,703	30,761	20,543
Income tax (benefit)									(5,942)	(4,415)	(2,989)
Income before equity in undistributed income of subsidiaries									38,645	35,176	23,532
Equity in undistributed income (loss) of subsidiaries									10,793	3,147	12,010
Net income attributable to common shareholders									49,438	38,323	35,542
Comprehensive income									$ 50,562	$ 28,778	$ 34,453