INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 11.4	$ 14.3	$ 22.7	$ 28.1
State income taxes net of federal tax benefit,amount	3.3	4.2	6.7	8.3
Federal excess deferred tax amortization, amount	(1.2)	7.5	(2.5)	5.9
Other, amount	(0.6)	(0.7)	(1.1)	(1.1)
Total income tax expense	$ 12.9	$ 25.3	$ 25.8	$ 41.2
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal income tax, percentage	21.00%	21.00%	21.00%	21.00%
State income taxes net of federal tax benefit, percent	6.10%	6.20%	6.20%	6.20%
Federal excess deferred tax amortization, percent	(2.20%)	11.10%	(2.30%)	4.40%
Other, percent	(1.20%)	(1.10%)	(1.10%)	(0.80%)
Total income tax expense, percentage	23.70%	37.20%	23.80%	30.80%