Income Taxes - Statutory rate reconciliation (Details) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018 USD ($) change	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected tax at statutory federal tax rates		$ 258.1	$ 555.5	$ 526.4
State income taxes net of federal tax benefit		71.8	100.8	72.8
Tax repairs		(120.7)	0.0	0.0
Federal excess amortization		(16.8)	0.0	0.0
Production tax credits		(12.1)	(16.8)	(15.7)
AFUDC - Equity		(3.2)	(4.0)	(8.8)
Investment tax credit restored		(2.8)	(2.7)	(4.9)
Federal tax reform		0.0	(226.9)	0.0
Other, net		(4.5)	(22.4)	(3.3)
Total income tax expense		$ 169.8	383.5	$ 566.5
Net impact of tax cuts and jobs act of 2017			$ 206.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected tax at statutory federal tax rates	21.00%	21.00%	35.00%	35.00%
State income taxes net of federal tax benefit		5.80%	6.40%	4.80%
Tax repairs		(9.80%)	0.00%	0.00%
Federal excess amortization		(1.40%)	0.00%	0.00%
Production tax credits		(1.00%)	(1.10%)	(1.10%)
AFUDC - Equity		(0.30%)	(0.30%)	(0.60%)
Investment tax credit restored		(0.20%)	(0.20%)	(0.30%)
Feceral tax reform		0.00%	(14.30%)	0.00%
Other, net		(0.30%)	(1.40%)	(0.20%)
Total income tax expense		13.80%	24.10%	37.60%
2018 and 2019 Rates | WE | Public Service Commission of Wisconsin (PSCW)
Income taxes
Income Statement impact of flow through of repair related deferred tax liabilities | change		0