INCOME TAXES (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount		$ 59.2		$ 159.7
State income taxes net of federal tax benefit, amount		17.7		47.6
Tax repairs, amount		(22.5)		(48.0)
Federal tax reform, amount		1.5		(14.0)
Other, amount		(4.8)		(5.9)
Total income tax expense, amount		$ 51.1	$ 115.8	$ 139.4	$ 329.1
Effective Income Tax Rate Reconciliation, Percent
Statutory federal income tax, percentage		21.00%		21.00%	35.00%
State income taxes net of federal tax benefit, percentage		6.30%		6.30%
Tax repairs, percentage		(8.00%)		(6.30%)
Federal tax reform, percentage		0.50%		(1.80%)
Other, percentage		(1.70%)		(0.90%)
Total income tax expense, percentage		18.10%		18.30%
Measurement period to complete accounting related to Tax Cuts and Jobs Act	1 year