Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits			$ 8.4		$ 12.0
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%	21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent			3.70%	3.70%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent			(8.50%)	(7.70%)
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	[1]		(4.60%)	(3.60%)
Regulatory differences - reversal of prior quarters' ARAM deferral (b)			(1.10%)	(1.30%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent			(0.80%)	(0.60%)
Effective Income Tax Rate Reconciliation, Percent			9.70%	11.50%
Federal income tax adjustment			$ 0.0
State income tax adjustment			0.0
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions			7.1		7.4
Unrecognized tax benefit — Temporary tax positions			1.3		4.6
Total unrecognized tax benefit			8.4		12.0
NOL and tax credit carryforwards			(7.0)		(8.3)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards			(5.7)		(5.0)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible			2.8
Amounts accrued for penalties related to unrecognized tax benefits			0.0		$ 0.0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Payable for interest related to unrecognized tax benefits at beginning of period		$ (1.1)	(1.1)	$ (0.7)
Interest benefit (expense) related to unrecognized tax benefits		$ (0.4)	(1.1)
Payable for interest related to unrecognized tax benefits at end of period			$ 0.0

[1]     Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.