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Summary of Significant Accounting Policies - Estimated Service Lives (Details)	12 Months Ended
	Dec. 31, 2020
Equipment | Minimum | Electric Plant in Service
Public Utility, Property, Plant and Equipment [Line Items]
Production, useful life	9 years
Transmission, useful life	51 years
Distribution, useful life	15 years
General, useful life	5 years
Equipment | Minimum | Nonelectric Property, Plant and Equipment
Public Utility, Property, Plant and Equipment [Line Items]
Useful life	2 years
Equipment | Maximum | Electric Plant in Service
Public Utility, Property, Plant and Equipment [Line Items]
Production, useful life	82 years
Transmission, useful life	75 years
Distribution, useful life	70 years
General, useful life	55 years
Equipment | Maximum | Nonelectric Property, Plant and Equipment
Public Utility, Property, Plant and Equipment [Line Items]
Useful life	12 years
Buildings and Leasehold Improvements | Minimum | Nonelectric Property, Plant and Equipment
Public Utility, Property, Plant and Equipment [Line Items]
Useful life	5 years
Buildings and Leasehold Improvements | Maximum | Nonelectric Property, Plant and Equipment
Public Utility, Property, Plant and Equipment [Line Items]
Useful life	40 years

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- Definition

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Useful life of utility distribution assets owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Useful life of utility transmission equipment owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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