INCOME TAXES - SUMMARY OF INCOME TAX EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Provision for income taxes
Current tax expense (benefit)	$ (66.1)	$ 56.7	$ 22.0
Deferred income taxes, net	125.5	20.9	78.0
Investment tax credit, net	(0.3)	(0.3)	(0.3)
Total income tax expense	$ 59.1	$ 77.3	$ 99.7