INCOME TAXES - Schedule of Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 1.1	$ 1.2	$ (16.0)
State	0.2	0.2	(2.0)
Total current expense (benefit)	1.3	1.4	(18.0)
Deferred:
Federal	39.8	29.9	48.8
State	5.4	5.9	4.2
Investment tax credits		(0.1)	(0.2)
Total deferred expense	45.2	35.7	52.8
Total income tax expense	$ 46.5	$ 37.1	$ 34.8