INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 20.1	$ 15.1	$ 50.5	$ 37.8
State income taxes net of federal tax benefit,amount	5.9	5.1	14.7	11.8
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(4.3)	0.0	(11.0)	0.0
Federal excess deferred tax amortization, amount	(1.8)	(2.5)	(4.5)	(5.0)
Other, amount	(0.3)	(0.7)	(1.3)	(1.8)
Total income tax expense	$ 19.6	$ 17.0	$ 48.4	$ 42.8
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%	7.10%	6.10%	6.50%
Federal excess deferred tax amortization - WI Unprotected, percent	(4.50%)	0.00%	(4.60%)	0.00%
Federal excess deferred tax amortization, percent	(1.90%)	(3.40%)	(1.90%)	(2.80%)
Other, percent	(0.40%)	(1.10%)	(0.50%)	(0.90%)
Total income tax expense, percent	20.40%	23.60%	20.10%	23.80%