INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 101.9	$ 100.5
State income taxes net of federal tax benefit, amount	31.0	29.9
Tax repairs, amount	(29.6)	(25.5)
Wind production tax credit, amount	(13.4)	(3.8)
Federal excess deferred tax amortization, amount	(13.2)	(15.5)
Excess tax benefits-stock options, amount	(7.2)	(0.9)
Other, amount	(4.5)	3.6
Total income tax expense, amount	$ 65.0	$ 88.3
Effective Income Tax Rate Reconciliation, Percent
Statutory federal income tax, percentage	21.00%	21.00%
State income taxes net of federal tax benefit, percentage	6.40%	6.20%
Tax repairs, percentage	(6.10%)	(5.30%)
Wind production tax credits, percent	(2.80%)	(0.80%)
Federal excess deferred tax amortization, percentage	(2.70%)	(3.20%)
Excess tax benefits-stock options, percent	(1.50%)	(0.20%)
Other, percentage	(0.90%)	0.70%
Total income tax expense, percentage	13.40%	18.40%