INCOME TAXES - Schedule of Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 1.2	$ (16.0)	$ (11.9)
State	0.2	(2.0)	(1.9)
Total current expense (benefit)	1.4	(18.0)	(13.8)
Deferred:
Federal	29.9	48.8	42.4
State	5.9	4.2	3.6
Investment tax credits	(0.1)	(0.2)	(0.2)
Total deferred expense	35.7	52.8	45.8
Total income tax expense	$ 37.1	$ 34.8	$ 32.0