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Note 13 - Property, Plant and Equipment - Estimated Service Lives for Properties (Details)	12 Months Ended
	Dec. 31, 2018
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)	42 years
Minimum [Member] \| Electric Plant [Member] \| Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	5 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)	70 years
Maximum [Member] \| Electric Plant [Member] \| Distribution Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	68 years
Maximum [Member] \| Electric Plant [Member] \| General Plant [Member]
Property, Plant and Equipment, Useful Life (Year)	50 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