PARENT ONLY CONDENSED FINANCIAL STATEMENTS - Parent Only Condensed Statement Of Cash Flows - (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
Operating Activities
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
Change in other assets and liabilities									(3,863)	(2,665)	1,402
Net cash provided by operating activities									65,918	47,035	53,392
Investing Activities
Proceeds from sales of securities available for sale									59,943	88,203	147,109
Cash received/(paid) from business acquisitions, net									2,845	(11,289)	57,083
Purchases of equipment									(4,347)	(14,068)	(7,794)
Net cash used by investing activities									(17,989)	(179,227)	(171,539)
Financing Activities
Proceeds from exercise of stock options									1,143	1,783	294
Proceeds from term debt										30,000
Purchase of treasury shares									(395)	(1,045)	(3,527)
Repayment on term debt									(6,666)	(11,667)
Cash dividends on common stock									(17,162)	(14,296)	(11,680)
Net cash provided by financing activities									(36,976)	151,166	123,240
Net change in cash and cash equivalents									10,953	18,974	5,093
Cash and cash equivalents, beginning of period				86,552				67,578	86,552	67,578	62,485
Cash and cash equivalents, end of period	97,505				86,552				97,505	86,552	67,578
MainSource Financial Group, Inc.
Operating Activities
Net income attributable to common shareholders									49,438	38,323	35,542
Undistributed (income)/loss of subsidiaries									(10,793)	(3,147)	(12,010)
Change in other assets and liabilities									(5,556)	(1,953)	(1,896)
Net cash provided by operating activities									44,201	37,129	25,428
Investing Activities
Proceeds from sales of securities available for sale										19
Cash received/(paid) from business acquisitions, net									(10,581)	(45,188)
Purchases of equipment									(233)	(1,215)	(1,991)
Net cash used by investing activities									(10,814)	(46,384)	(1,991)
Financing Activities
Proceeds from exercise of stock options									1,143	1,783	294
Proceeds from term debt										30,000
Purchase of treasury shares									(395)	(1,045)	(3,527)
Repayment on term debt									(6,666)	(11,667)
Cash dividends on common stock									(17,162)	(14,296)	(11,680)
Net cash provided by financing activities									(23,080)	4,775	(14,913)
Net change in cash and cash equivalents									10,307	(4,480)	8,524
Cash and cash equivalents, beginning of period				$ 10,366				$ 14,846	10,366	14,846	6,322
Cash and cash equivalents, end of period	$ 20,673				$ 10,366				$ 20,673	$ 10,366	$ 14,846