Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Electricity [Member]
Net revenue from external customers	$ 109,795	$ 97,245	$ 321,664	$ 278,124
Intersegment revenue
Operating income	23,903	28,346	91,502	73,220
Segment assets at period end	2,137,845	2,103,754	2,137,845	2,103,754
Product Segment [Member]
Net revenue from external customers	74,822	65,607	174,408	145,446
Intersegment revenue	14,835	10,658	36,042	41,314
Operating income	24,320	18,133	59,151	41,753
Segment assets at period end	141,426	182,579	141,426	182,579
Net revenue from external customers	184,617	162,852	496,072	423,570
Intersegment revenue	14,835	10,658	36,042	41,314
Operating income	48,223	46,479	150,653	114,973
Segment assets at period end	$ 2,279,271	$ 2,286,333	$ 2,279,271	$ 2,286,333	$ 2,273,982