Note 13 - Property, Plant and Equipment (Details Textual)	12 Months Ended
	Dec. 31, 2018
Electric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years
Electric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	82 years
Nonelectric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	2 years
Nonelectric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years