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	$ 32.3	$ 28.1	$ 82.2	$ 72.2
State income taxes net of federal tax benefit, amount	9.8	8.6	25.0	22.1
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(12.5)	(11.4)	(35.8)	(34.3)
Federal excess deferred tax amortization, amount	(6.8)	(6.2)	(19.4)	(18.6)
PTCs, amount	(1.9)	(2.9)	(5.7)	(8.6)
Domestic production activities deferral, Amount	1.9	(0.9)	5.3	2.5
Other, amount	1.6	3.8	4.5	4.5
Total income tax expense, amount	$ 24.4	$ 19.1	$ 56.1	$ 39.8
Effective Income Tax Rate Reconciliation, Percent
Statutory federal income tax, percent	21.00%	21.00%	21.00%	21.00%
State income taxes net of federal tax benefit, percent	6.40%	6.40%	6.40%	6.40%
Federal excess deferred tax amortization - WI Unprotected, percent	(8.10%)	(8.50%)	(9.10%)	(10.00%)
Federal excess deferred tax amortization, percent	(4.40%)	(4.70%)	(5.00%)	(5.40%)
PTCs, percent	(1.20%)	(2.20%)	(1.50%)	(2.50%)
Domestic production activities deferral, percent	1.20%	(0.70%)	1.40%	0.70%
Other, percent	0.90%	3.00%	1.10%	1.30%
Total income tax expense, percent	15.80%	14.30%	14.30%	11.50%