Segments Of Business (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2019 USD ($)	Dec. 31, 2020
Goodwill [Line Items]
Number of Reportable Segments		2
Number of counties in which electric service provided by Electric Operations		20
Columbia Of Massachusetts
Goodwill [Line Items]
Goodwill, Impairment Loss	$ 204.8

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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