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
Components of deferred income taxes classified as net current assets and net long-term liabilities [Abstract]
Statutory federal income tax	21.00%		21.00%	21.00%	35.00%
Estimated tax benefit related to the remeasurement of deferred income taxes from tax legislation		$ 2,657.0
Net impact of tax cuts and jobs act of 2017					$ 206.7
Non-current
Tax gross up - regulatory items			$ 519.8	$ 579.2
Deferred revenues			106.3	129.3
Future tax benefits			101.0	70.6
Other			159.8	194.4
Total deferred tax assets			886.9	973.5
Valuation allowance			(2.3)	(11.4)
Net deferred tax assets			884.6	962.1
Non-current
Property-related			3,609.0	3,436.9
Investment in affiliates			531.7	420.6
Deferred costs - Plant retirements			232.0	176.0
Employee benefits and compensation			131.4	121.2
Other			149.8	195.5
Total deferred tax liabilities			4,653.9	4,350.2
Deferred tax liability, net			$ 3,769.3	$ 3,388.1