Note 9 - Business Segments (Details) - Summarized Financial Information Concerning Reportable Segments - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 140,487	$ 127,603	$ 260,718	$ 270,039
Intersegment revenues	140,487	127,603	260,718	270,039
Operating income	38,643	22,330	68,494	64,901
Segment assets at period end *	2,276,550	2,144,929	2,276,550	2,144,929	$ 2,121,556
* Including unconsolidated investments	(5,215)	3,308	(5,215)	3,308	$ (3,617)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	10,900	15,742	30,657	36,336
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers	90,926	91,692	180,879	186,509
Operating income	20,920	9,521	44,874	40,439
Segment assets at period end *	2,080,209	2,027,211	2,080,209	2,027,211
* Including unconsolidated investments		3,308		3,308
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers	49,561	35,911	79,839	83,530
Operating income	17,723	12,809	23,620	24,462
Segment assets at period end *	196,341	117,718	196,341	117,718
Product Segment [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	$ 10,900	$ 15,742	$ 30,657	$ 36,336