INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 69.1	$ 62.2	$ 191.9	$ 176.1
State income taxes net of federal tax benefit, amount	20.8	18.4	57.7	52.4
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(16.3)	(11.1)	(46.6)	(33.2)
PTCs, amount	(13.5)	(9.4)	(47.5)	(27.8)
Federal excess deferred tax amortization, amount	(7.9)	(8.9)	(22.5)	(21.9)
Other, amount	1.9	2.6	(4.0)	(1.8)
Total income tax expense, amount	$ 54.1	$ 53.8	$ 129.0	$ 143.8
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%	6.20%	6.30%	6.30%
Federal excess deferred tax amortization - Wisconsin unprotected, percentage	(5.00%)	(3.80%)	(5.10%)	(4.00%)
PTCs, percentage	(4.10%)	(3.20%)	(5.20%)	(3.30%)
Federal excess deferred tax amortization, percentage	(2.40%)	(3.00%)	(2.50%)	(2.60%)
Other, percentage	0.60%	1.00%	(0.40%)	(0.30%)
Total income tax expense, percent	16.40%	18.20%	14.10%	17.10%