Income Taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Non-current
Tax gross up - regulatory items		$ 585.8	$ 579.2	$ 585.8
Deferred revenues		128.8	129.3	128.8
Future tax benefits		303.9	70.6	303.9
Employee benefits and compensation		164.2	0.0	164.2
Property-related		24.4	0.0	24.4
Other		185.0	194.4	185.0
Total deferred tax assets		1,392.1	973.5	1,392.1
Valuation allowance		(15.7)	(11.4)	(15.7)
Net deferred tax assets		1,376.4	962.1	1,376.4
Non-current
Property-related		3,464.6	3,436.9	3,464.6
Investment in transmission affiliate		321.2	420.6	321.2
Deferred costs - Pleasant Prairie		0.0	176.0	0.0
Employee benefits and compensation		285.8	121.2	285.8
Deferred transmission costs		60.1	55.4	60.1
Other		244.5	140.1	244.5
Total deferred tax liabilities		4,376.2	4,350.2	4,376.2
Deferred tax liability, net		2,999.8	$ 3,388.1	2,999.8
Net impact of tax cuts and jobs act of 2017				$ 206.7
Expected tax at statutory federal tax rates	21.00%		21.00%	35.00%	35.00%
Estimated tax benefit related to the remeasurement of deferred taxes from tax legislation		$ 2,657.0