Note 9 - Business Segments (Details) - Summarized Financial Information Concerning Reportable Segments (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Mar. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 130,749		$ 132,297		$ 402,334		$ 388,453
Operating income	29,797		12,065		75,343		61,047
Segment assets at period end *	2,168,335	[1]	2,258,363	[1]	2,168,335	[1]	2,258,363	[1]	2,133,925	2,087,523
* Including unconsolidated investments	5,419		3,476		5,419		3,476			2,591
Intersegment Eliminations [Member] | Product Segment [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	4,329		11,063		29,731		32,970
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	4,329		11,063		29,731		32,970
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers	88,994		77,612		245,005		238,837
Operating income	20,732		4,681		42,057		35,980
Segment assets at period end *	2,048,021	[1]	2,150,533	[1]	2,048,021	[1]	2,150,533	[1]
* Including unconsolidated investments	5,419		2,496		5,419		2,496
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers	41,755		54,685		157,329		149,616
Operating income	9,065		7,384		33,286		25,067
Segment assets at period end *	120,314	[1]	107,830	[1]	120,314	[1]	107,830	[1]
* Including unconsolidated investments			$ 980				$ 980

[1] Including unconsolidated investments