PARENT ONLY CONDENSED FINANCIAL STATEMENTS - Parent Only Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 97,505	$ 86,552	$ 67,578	$ 62,485
Securities available for sale	1,077,573	1,007,540
Total assets	4,647,862	4,080,257
Liabilities
Subordinated debentures	56,545	41,239
Borrowings	11,667	18,333
Other liabilities	29,118	19,323
Total liabilities	4,114,904	3,630,763
Shareholders' equity	532,958	449,494	381,360	360,662
Total liabilities and shareholders' equity	4,647,862	4,080,257
MainSource Financial Group, Inc.
Assets
Cash and cash equivalents	20,673	10,366	$ 14,846	$ 6,322
Securities available for sale	130	130
Investment in subsidiaries	582,300	494,158
Other assets	7,069	8,396
Total assets	610,172	513,050
Liabilities
Subordinated debentures	58,545	41,239
Borrowings	11,667	18,333
Other liabilities	7,002	3,984
Total liabilities	77,214	63,556
Shareholders' equity	532,958	449,494
Total liabilities and shareholders' equity	$ 610,172	$ 513,050