QUARTERLY FINANCIAL DATA - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
After-tax merger and conversion charges	$ 15.1
After-tax executive retirement benefits costs	$ 4.5
Tax reform, additional tax expense from remeasurement from reduced tax rate		$ 13.6
After-tax compliance resolution expense			$ 3.1