INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 71.8	$ 65.9	$ 263.7	$ 242.0
State income taxes net of federal tax benefit, amount	21.5	19.7	79.2	72.1
PTCs, amount	(17.0)	(11.2)	(64.5)	(39.0)
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(16.3)	(12.5)	(62.9)	(45.7)
Federal excess deferred tax amortization, amount	(7.8)	(8.3)	(30.3)	(30.2)
Other, amount	(1.4)	(6.7)	(5.4)	(8.5)
Total income tax expense, amount	$ 50.8	$ 46.9	$ 179.8	$ 190.7
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%)	(3.60%)	(5.10%)	(3.40%)
Federal excess deferred tax amortization - Wisconsin unprotected, percentage	(4.80%)	(4.00%)	(5.00%)	(4.00%)
Federal excess deferred tax amortization, percentage	(2.30%)	(2.60%)	(2.40%)	(2.60%)
Other, percentage	(0.20%)	(2.20%)	(0.50%)	(0.80%)
Total income tax expense, percent	15.00%	14.90%	14.30%	16.50%