Income Taxes (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2017
Income Tax [Line Items]
Reduction in net deferred income tax liabilities		$ 1,140.0
Amortization of an excess deferred tax liability	$ 13.5
Regulatory liability, amortization period	28 years 6 months
Domestic Tax Authority
Income Tax [Line Items]
Amortization of an excess deferred tax liability	$ 14.0