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	$ 52.7	$ 59.2	$ 154.6	$ 159.7
State income taxes net of federal tax benefit, amount	15.6	17.7	46.5	47.6
Tax repairs, amount	(30.4)	(22.5)	(60.0)	(48.0)
Federal excess deferred tax amortization, amount	(7.5)	1.5	(20.7)	(14.0)
Wind production tax credit, amount	(6.2)	(2.1)	(19.6)	(5.9)
Excess tax benefits-stock options, amount	(4.4)	(1.0)	(11.6)	(1.9)
Other, amount	(4.6)	(1.7)	(9.0)	1.9
Total income tax expense, amount	$ 15.2	$ 51.1	$ 80.2	$ 139.4
Effective Income Tax Rate Reconciliation, Percent
Statutory federal income tax, percentage	21.00%	21.00%	21.00%	21.00%
State income taxes net of federal tax benefit, percentage	6.20%	6.30%	6.30%	6.30%
Tax repairs, percentage	(12.10%)	(8.00%)	(8.10%)	(6.30%)
Federal excess deferred tax amortization, percentage	(3.00%)	0.50%	(2.80%)	(1.80%)
Wind production tax credits, percent	(2.50%)	(0.70%)	(2.70%)	(0.80%)
Excess tax benefits-stock options, percent	(1.70%)	(0.30%)	(1.60%)	(0.30%)
Other, percentage	(1.80%)	(0.70%)	(1.20%)	0.20%
Total income tax expense, percentage	6.10%	18.10%	10.90%	18.30%