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Note 13 - Property, Plant and Equipment - Estimated Service Lives for Properties (Details)	12 Months Ended
	Dec. 31, 2019
Minimum [Member] | Electric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Minimum [Member] | Electric Plant [Member] | Electric Generation Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	9 years
Minimum [Member] | Electric Plant [Member] | Transmission Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	51 years
Minimum [Member] | Electric Plant [Member] | Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	15 years
Minimum [Member] | Electric Plant [Member] | General Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Minimum [Member] | Nonelectric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	2 years
Minimum [Member] | Nonelectric Plant [Member] | Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	2 years
Minimum [Member] | Nonelectric Plant [Member] | Buildings and Leasehold Improvements [Member]
Property, Plant and Equipment, Useful Life (Year)	5 years
Maximum [Member] | Electric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	82 years
Maximum [Member] | Electric Plant [Member] | Electric Generation Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	82 years
Maximum [Member] | Electric Plant [Member] | Transmission Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	75 years
Maximum [Member] | Electric Plant [Member] | Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	70 years
Maximum [Member] | Electric Plant [Member] | General Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	57 years
Maximum [Member] | Nonelectric Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	40 years
Maximum [Member] | Nonelectric Plant [Member] | Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)	12 years
Maximum [Member] | Nonelectric Plant [Member] | Buildings and Leasehold Improvements [Member]
Property, Plant and Equipment, Useful Life (Year)	40 years

X

- Definition

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.

+ References

No definition available.

+ Details

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- Details

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