INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 78.9	$ 71.8	$ 298.1	$ 263.7
State income taxes net of federal tax benefit, amount	23.5	21.5	89.3	79.2
PTCs, amount	(18.6)	(17.0)	(86.3)	(64.5)
Federal excess deferred tax amortization, amount	(8.1)	(7.8)	(32.3)	(30.3)
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(0.1)	(16.3)	(0.6)	(62.9)
Other, amount	(2.2)	(1.4)	(4.3)	(5.4)
Total income tax expense, amount	$ 73.4	$ 50.8	$ 263.9	$ 179.8
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	6.30%	6.30%	6.30%	6.30%
PTCs, percentage	(5.00%)	(5.00%)	(6.10%)	(5.10%)
Federal excess deferred tax amortization, percentage	(2.20%)	(2.30%)	(2.30%)	(2.40%)
Federal excess deferred tax amortization - Wisconsin unprotected, percentage	0.00%	(4.80%)	0.00%	(5.00%)
Other, percentage	(0.50%)	(0.20%)	(0.30%)	(0.50%)
Total income tax expense, percent	19.60%	15.00%	18.60%	14.30%