INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 62.2	$ 52.7	$ 176.1	$ 154.6
State income taxes net of federal tax benefit, amount	18.4	15.6	52.4	46.5
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(11.1)	0.0	(33.2)	0.0
Wind production tax credits, amount	(9.4)	(6.2)	(27.8)	(19.6)
Federal excess deferred tax amortization, amount	(8.9)	(7.5)	(21.9)	(20.7)
Excess tax benefits-stock options, amount	(1.4)	(4.4)	(6.3)	(11.6)
Tax repairs, amount	0.0	(30.4)	1.5	(60.0)
Other, amount	4.0	(4.6)	3.0	(9.0)
Total income tax expense, amount	$ 53.8	$ 15.2	$ 143.8	$ 80.2
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	6.20%	6.20%	6.30%	6.30%
Federal excess deferred tax amortization - Wisconsin unprotected, percentage	(3.80%)	0.00%	(4.00%)	0.00%
Wind production tax credits, percentage	(3.20%)	(2.50%)	(3.30%)	(2.70%)
Federal excess deferred tax amortization, percentage	(3.00%)	(3.00%)	(2.60%)	(2.80%)
Excess tax benefits-stock options, percentage	(0.50%)	(1.70%)	(0.80%)	(1.60%)
Tax repairs, percentage	0.00%	(12.10%)	0.20%	(8.10%)
Other, percentage	1.50%	(1.80%)	0.30%	(1.20%)
Total income tax expense, percentage	18.20%	6.10%	17.10%	10.90%