Income Taxes - Statutory rate reconciliation (Details) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019 USD ($) change	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory federal income tax		$ 63.9	$ 63.3	$ 184.4
State income taxes net of federal tax benefit		20.2	19.6	27.9
Tax repairs		(122.9)	(120.7)	0.0
Federal excess amortization		(16.1)	(15.5)	0.0
Wind production tax credits		(9.3)	(9.4)	(17.6)
Investment tax credit restored		(2.3)	(0.8)	(0.9)
AFUDC - Equity		(0.8)	(0.8)	(1.1)
Domestic production activities deferral		6.1	6.1
Domestic production activities (deduction)				(7.8)
Other, net		3.4	1.3	6.3
Total income tax expense (benefit)		$ (57.8)	$ (56.9)	$ 191.2
Effective Income Tax Rate Reconciliation, Percentage [Abstract]
Statutory federal income tax	21.00%	21.00%	21.00%	35.00%
State income taxes net of federal tax benefit		6.60%	6.50%	5.30%
Tax repairs		(40.10%)	(39.90%)	0.00%
Federal excess amortization		(5.30%)	(5.10%)	0.00%
Wind production tax credits		(3.00%)	(3.10%)	(3.30%)
Investment tax credit restored		(0.80%)	(0.30%)	(0.20%)
AFUDC - Equity		(0.30%)	(0.30%)	(0.20%)
Domestic production activities deferral		2.00%	2.00%
Domestic production activities (deduction)				(1.50%)
Other, net		1.00%	0.40%	1.10%
Total income tax expense (benefit)		(18.90%)	(18.80%)	36.20%
2018 and 2019 rates | Public Service Commission of Wisconsin (PSCW)
Income Taxes
Income statement impact of flow through of repair related deferred tax liabilities | change		0