INCOME TAXES - STATUTORY RATE RECONCILIATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Reconciliation of federal income taxes to the provision for income taxes reported in the income statement
Statutory federal income tax	$ 67.8	$ 64.5	$ 55.1
State income taxes net of federal tax benefit	20.2	19.5	16.3
PTCs, net	(14.4)	(0.6)	0.0
Federal Excess deferred tax amortization	(5.7)	(5.2)	(5.2)
Federal excess deferred tax amortization - Wisconsin unprotected	(3.8)	(3.8)	(33.0)
ITCs	(3.8)	(2.8)	(1.6)
Other, net	2.4	0.6	(0.4)
Total income tax expense	$ 62.7	$ 72.2	$ 31.2
Reconciliation of federal income taxes to the provision for income taxes reported in the income statement (as a percent)
Statutory federal income tax	21.00%	21.00%	21.00%
State income taxes net of federal tax benefit	6.30%	6.30%	6.20%
PTCs, net	(4.50%)	(0.20%)	0.00%
Federal excess deferred tax amortization	(1.80%)	(1.70%)	(2.00%)
Federal excess deferred tax amortization - Wisconsin unprotected	(1.20%)	(1.20%)	(12.60%)
ITCs	(1.20%)	(0.90%)	(0.60%)
Other, net	0.80%	0.20%	(0.10%)
Total income tax expense	19.40%	23.50%	11.90%
2018 and 2019 rates | Public Service Commission of Wisconsin (PSCW) | Tax Cuts and Jobs Act of 2017
Income taxes
Income Statement Impact of amortizing unprotected tax benefits	$ 0.0	$ 0.0	$ 0.0
2020 and 2021 rates | Public Service Commission of Wisconsin (PSCW) | Tax Cuts and Jobs Act of 2017
Income taxes
Income Statement Impact of amortizing protected tax benefits	$ 0.0	$ 0.0	$ 0.0
2020 and 2021 rates | Public Service Commission of Wisconsin (PSCW) | Electric rates | Tax Cuts and Jobs Act of 2017
Income taxes
Amortization period				2 years
2020 and 2021 rates | Public Service Commission of Wisconsin (PSCW) | Natural gas rates | Tax Cuts and Jobs Act of 2017
Income taxes
Amortization period				4 years