Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016	Dec. 31, 2016
Net revenues	$ 157,185		$ 184,617	$ 526,447		$ 496,072
Operating income	43,970		48,223	156,618		150,653
Segment assets at period end	2,503,738	[1]	2,279,271	2,503,738	[1]	2,279,271	$ 2,461,569
Intersegment Eliminations [Member]
Net revenues	28,248		14,835	61,026		36,042
Electricity [Member]
Net revenues	112,273		109,795	339,826		321,664
Operating income	36,205		23,903	113,220		91,502
Segment assets at period end	2,371,855	[1]	2,137,845	2,371,855	[1]	2,137,845
Electricity [Member] | Intersegment Eliminations [Member]
Net revenues
Product Segment [Member]
Net revenues	44,912		74,822	186,621		174,408
Operating income	7,765		24,320	43,398		59,151
Segment assets at period end	131,883	[1]	141,426	131,883	[1]	141,426
Product Segment [Member] | Intersegment Eliminations [Member]
Net revenues	$ 28,248		$ 14,835	$ 61,026		$ 36,042

[1] Electricity segment assets include goodwill in the amount of $20.7 million