Summary of Significant Accounting Policies Estimated average service lives (Details)	12 Months Ended
Dec. 31, 2020
Generation, excluding thermal:
Hydro	97 years
Wind	31 years
Transmission	58 years
Distribution	46 years
General	13 years

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.

No definition available.

Name:	us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration