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Nature of Operations And Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 09, 2020 USD ($)	Feb. 26, 2020 USD ($)
Basis Of Accounting Presentation [Line Items]
Number of customers			3,700,000
Net Proceeds from Sale						$ 1,113.0
Purchase Price							$ 1,100.0
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down			$ 412.4
Unbilled revenue, less reserves			338.3	$ 350.5
Other than Temporary Impairment Losses, Investments			$ 0.0	$ 0.0	$ 0.0
Pre-tax rate for allowance for funds used during construction			2.60%	3.00%	3.50%
Inventory valued using LIFO			$ 42.3	$ 47.2
Excess of replacement over LIFO value			(19.6)	(25.5)
Inventory valued using the weighted average cost methodology			148.8	203.7
Massachusetts Business [Domain]
Basis Of Accounting Presentation [Line Items]
Disposal Group, Including Discontinued Operation, Operating Income (Loss)			(422.3)	$ 36.8	$ (835.6)
Columbia Of Massachusetts
Basis Of Accounting Presentation [Line Items]
Settlement Agreement Payment in Lieu of Penalties			$ 56.0
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down	$ 84.4	$ 280.2

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