INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 145.5	$ 122.8
State income taxes net of federal tax benefit, amount	43.6	36.9
PTCs, amount	(44.8)	(34.0)
Federal excess deferred tax amortization, amount	(15.8)	(14.6)
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(0.3)	(30.3)
Uncertain tax positions, amount	0.0	(8.2)
Other, amount	(1.1)	2.3
Total income tax expense, amount	$ 127.1	$ 74.9
Effective Income Tax Rate Reconciliation, Percent
Statutory federal income tax, percentage	21.00%	21.00%
State income taxes net of federal tax benefit, percentage	6.30%	6.30%
PTCs, percentage	(6.50%)	(5.80%)
Federal excess deferred tax amortization, percentage	(2.30%)	(2.50%)
Federal excess deferred tax amortization - Wisconsin unprotected, percentage	0.00%	(5.20%)
Uncertain tax position, percentage	0.00%	(1.40%)
Other, percentage	(0.20%)	0.40%
Total income tax expense, percent	18.30%	12.80%