INCOME TAXES (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amount
Income tax provision at the statutory rate	$ 93.7	$ 95.7	$ 64.1
State income tax expense, net	7.8	8.0	5.8
Non-deductible expenses, net	0.2	0.3	1.2
Adjustments and settlements	(0.8)	0.0	(0.5)
Other, net	(0.3)	0.0	0.4
Total Income Tax Expense	$ 100.6	$ 104.0	$ 71.0
Percent
Income tax provision at the statutory rate	35.00%	35.00%	35.00%
State income tax expense, net	2.90%	2.90%	3.20%
Non-deductible expenses, net	0.10%	0.10%	0.70%
Adjustments and settlements	(0.30%)	0.00%	(0.30%)
Other, net	(0.10%)	0.00%	0.20%
Income tax provision	37.60%	38.00%	38.80%