Note 9 - Business Segments (Details) - Summarized Financial Information Concerning Reportable Segments - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 162,852	$ 140,242	$ 423,570	$ 410,281
Intersegment revenues	162,852	140,242	423,570	410,281
Operating income	46,479	43,788	114,973	108,689
Segment assets at period end *	2,286,333	2,171,913	2,286,333	2,171,913	$ 2,121,556
* Including unconsolidated investments	(12,667)	1,339	(12,667)	1,339	$ (3,617)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	10,657	7,244	41,314	43,580
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers	97,245	102,506	278,124	289,015
Operating income	28,346	32,411	73,220	72,850
Segment assets at period end *	2,103,754	2,083,715	2,103,754	2,083,715
* Including unconsolidated investments		1,339		1,339
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers	65,607	37,736	145,446	121,266
Operating income	18,133	11,377	41,753	35,839
Segment assets at period end *	182,579	88,198	182,579	88,198
Product Segment [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	$ 10,657	$ 7,244	$ 41,314	$ 43,580