Note 9 - Business Segments (Details) - Summarized Financial Information Concerning Reportable Segments (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 127,603	$ 152,679	$ 270,039	$ 271,585
Operating income	22,330	37,862	64,901	45,546
Segment assets at period end *	2,144,929	2,137,900	2,144,929	2,137,900	2,159,433
* Including unconsolidated investments	3,308	3,524	3,308	3,524	7,076
Intersegment Eliminations [Member] | Product Segment [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	15,742	18,821	36,336	25,402
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	15,742	18,821	36,336	25,402
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers	91,692	87,713	186,509	156,011
Operating income	9,521	22,639	40,439	21,325
Segment assets at period end *	2,027,211	2,039,219	2,027,211	2,039,219
* Including unconsolidated investments	3,308	3,524	3,308	3,524
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers	35,911	64,966	83,530	115,574
Operating income	12,809	15,223	24,462	24,221
Segment assets at period end *	$ 117,718	$ 98,681	$ 117,718	$ 98,681