BUSINESS COMBINATION Business Combination - Additional Information - Narrative (Details) - USD ($) $ in Thousands	Apr. 01, 2018	Mar. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Business Acquisition, Number Of Shares Received by Acquiree	1.3875
Business Combination, Consideration Transferred	$ 1,000,000
Assets		$ 8,898,429	$ 8,896,923
Loans and Leases Receivable, Net Amount		6,047,641	5,959,162
Deposits		7,010,504	$ 6,895,046
MainSource [Member]
Business Acquisition [Line Items]
Assets		4,600,000
Loans and Leases Receivable, Net Amount		3,000,000
Deposits		$ 3,400,000