INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Effective Income Tax Rate Reconciliation, Amount
Statutory federal income tax, amount	$ 122.8	$ 113.9
State income taxes net of federal tax benefit, amount	36.9	34.0
Wind production tax credits, amount	(34.0)	(18.4)
Federal excess deferred tax amortization - Wisconsin unprotected, amount	(30.3)	(22.1)
Federal excess deferred tax amortization, amount	(14.6)	(13.0)
Uncertain tax positions, amount	(8.2)	0.0
Other, amount	2.3	(4.4)
Total income tax expense, amount	$ 74.9	$ 90.0
Effective Income Tax Rate Reconciliation, Percent
Statutory federal income tax, percentage	21.00%	21.00%
State income taxes net of federal tax benefit, percentage	6.30%	6.30%
Wind production tax credits, percentage	(5.80%)	(3.40%)
Federal excess deferred tax amortization - Wisconsin unprotected, percentage	(5.20%)	(4.10%)
Federal excess deferred tax amortization, percentage	(2.50%)	(2.40%)
Uncertain tax position, percentage	(1.40%)	0.00%
Other, percentage	0.40%	(0.80%)
Total income tax expense, percentage	12.80%	16.60%