INCOME TAXES - STATUTORY RATE RECONCILIATION (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of federal income taxes to the provision for income taxes reported in the income statement
Statutory federal income tax			$ 51.2	$ 52.5	$ 89.1
State income taxes net of federal tax benefit			15.1	15.4	12.7
Federal excess amortization			(4.6)	11.6	0.0
AFUDC - Equity			(1.2)	(1.0)	(1.4)
Other, net			(1.4)	(1.2)	(0.7)
Total income tax expense			$ 59.1	$ 77.3	$ 99.7
Reconciliation of federal income taxes to the provision for income taxes reported in the income statement (as a percent)
Statutory federal income tax	21.00%	35.00%	21.00%	21.00%	35.00%
State income taxes net of federal tax benefit			6.20%	6.20%	5.00%
Federal excess amortization			(1.90%)	4.60%	0.00%
AFUDC - Equity			(0.50%)	(0.40%)	(0.50%)
Other, net			(0.60%)	(0.50%)	(0.30%)
Total income tax expense			24.20%	30.90%	39.20%