Income Taxes - Statutory Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Statutory rate reconciliation of amount
Statutory federal income tax	$ 322.6	$ 363.5	$ 315.1
State income taxes net of federal tax benefit	94.3	109.7	96.1
PTCs, net	(168.2)	(107.6)	(81.3)
Federal excess deferred tax amortization	(37.6)	(36.9)	(37.3)
AFUDC - Equity	(12.4)	(6.2)	(3.8)
Federal excess deferred tax amortization - Wisconsin unprotected	(0.8)	(0.8)	(77.9)
Other, net	6.7	1.2	(10.6)
Total income tax expense	$ 204.6	$ 322.9	$ 200.3
Statutory rate reconciliation of percent
Statutory federal income tax	21.00%	21.00%	21.00%
State income taxes net of federal tax benefit	6.10%	6.30%	6.40%
PTCs, net	(10.90%)	(6.20%)	(5.40%)
Federal excess deferred tax amortization	(2.40%)	(2.10%)	(2.50%)
AFUDC - Equity	(0.80%)	(0.40%)	(0.30%)
Federal excess deferred tax amortization - Wisconsin unprotected	(0.10%)	0.00%	(5.20%)
Other, net	0.40%	0.00%	(0.60%)
Total income tax expense	13.30%	18.60%	13.40%
Public Service Commission of Wisconsin (PSCW) | Tax Cuts and Jobs Act of 2017 | 2020 and 2021 rates
Income taxes
Income statement impact of amortizing unprotected tax benefits	$ 0.0	$ 0.0	$ 0.0
Public Service Commission of Wisconsin (PSCW) | Tax Cuts and Jobs Act of 2017 | 2020 and 2021 rates | Electric rates
Income taxes
Amortization period				2 years
Public Service Commission of Wisconsin (PSCW) | Tax Cuts and Jobs Act of 2017 | 2020 and 2021 rates | Natural gas rates
Income taxes
Amortization period				4 years
Public Service Commission of Wisconsin (PSCW) | Tax Cuts and Jobs Act of 2017 | 2018 and 2019 rates
Income taxes
Income statement impact of amortizing protected tax benefits	$ 0.0	$ 0.0	$ 0.0