Income Taxes - Statutory rate reconciliation (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory federal income tax		$ 299.9	$ 264.4	$ 258.1
State income taxes net of federal tax benefit		90.5	80.4	71.8
Federal excess deferred tax amortization – Wisconsin unprotected (1)		(57.6)	0.0	0.0
Wind production tax credits		(51.5)	(34.1)	(12.1)
Federal excess deferred tax amortization		(36.7)	(34.9)	(16.8)
Excess tax benefits - stock options		(12.3)	(15.8)	(5.9)
AFUDC - Equity		(4.4)	(3.0)	(3.2)
Investment tax credit restored		(3.5)	(4.8)	(2.8)
Tax repairs		3.3	(122.8)	(120.7)
Other, net		0.2	(4.4)	1.4
Total income tax expense		$ 227.9	$ 125.0	$ 169.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal income tax		21.00%	21.00%	21.00%
State income taxes net of federal tax benefit		6.30%	6.40%	5.80%
Federal excess deferred tax amortization - Wisconsin unprotected		(4.00%)	0.00%	0.00%
Wind production tax credits		(3.60%)	(2.70%)	(1.00%)
Federal excess deferred tax amortization		(2.60%)	(2.80%)	(1.40%)
Excess tax benefits - stock options		(0.90%)	(1.30%)	(0.50%)
AFUDC - Equity		(0.30%)	(0.20%)	(0.30%)
Investment tax credit restored		(0.20%)	(0.40%)	(0.20%)
Tax repairs		0.20%	(9.80%)	(9.80%)
Other, net		0.00%	(0.30%)	0.20%
Total income tax expense		15.90%	9.90%	13.80%
2018 and 2019 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
2018 and 2019 rates | Public Service Commission of Wisconsin (PSCW) | WE | Tax Repairs Regulatory Asset
Income taxes
Income statement impact of flow through of repair related deferred tax liabilities		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 unprotected tax benefits		$ 0.0
2020 and 2021 rates | Public Service Commission of Wisconsin (PSCW) | WE | Tax Repairs Regulatory Asset
Income taxes
Amortization period		50 years
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	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	4 years