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	$ 17.3	$ 20.1	$ 48.1	$ 50.5
State income taxes net of federal tax benefit,amount	5.1	5.9	14.3	14.7
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(9.8)	(4.3)	(28.7)	(11.0)
Federal excess deferred tax amortization, amount	(1.6)	(1.8)	(4.5)	(4.5)
Other, amount	(0.6)	(0.3)	(1.6)	(1.3)
Total income tax expense, amount	$ 10.4	$ 19.6	$ 27.6	$ 48.4
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal income tax, percent	21.00%	21.00%	21.00%	21.00%
State income taxes net of federal tax benefit, percent	6.20%	6.20%	6.20%	6.10%
Federal excess deferred tax amortization - WI Unprotected, percent	(11.90%)	(4.50%)	(12.50%)	(4.60%)
Federal excess deferred tax amortization, percent	(1.90%)	(1.90%)	(2.00%)	(1.90%)
Other, percent	(0.80%)	(0.40%)	(0.60%)	(0.50%)
Total income tax expense, percent	12.60%	20.40%	12.10%	20.10%