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, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory federal income tax	21.00%		21.00%	21.00%	35.00%
Estimated tax benefit related to the remeasurement of deferred taxes from tax legislation		$ 1,065.0
Non-current
Tax gross up - regulatory items			$ 152.7	$ 203.0
Deferred revenues			126.8	129.3
Future tax benefits			41.0	15.9
Regulatory deferrals			1.3	97.3
Other			63.2	59.3
Total deferred tax assets			385.0	504.8
Non-current
Property-related			1,368.9	1,365.9
Deferred costs - Plant retirements			215.5	176.0
Employee benefits and compensation			55.8	55.9
Deferred costs - SSR			51.1	110.7
Other			41.1	94.6
Total deferred tax liabilities			1,732.4	1,803.1
Deferred tax liability, net			$ 1,347.4	$ 1,298.3