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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 140,242		$ 130,749		$ 410,281		$ 402,334
Operating income	43,788		29,797		108,689		75,343
Segment assets at period end *	2,171,913	[1]	2,168,335	[1]	2,171,913	[1]	2,168,335	[1]	2,159,433
* Including unconsolidated investments	1,339	[1]	5,419	[1]	1,339	[1]	5,419	[1]	7,076
Intersegment Eliminations [Member] | Product Segment [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	7,244		4,329		43,580		29,731
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	7,244		4,329		43,580		29,731
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers	102,506		88,994		289,015		245,005
Operating income	32,411		20,732		72,850		42,057
Segment assets at period end *	2,083,715	[1]	2,048,021	[1]	2,083,715	[1]	2,048,021	[1]
* Including unconsolidated investments	1,339	[1]	5,419	[1]	1,339	[1]	5,419	[1]
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers	37,736		41,755		121,266		157,329
Operating income	11,377		9,065		35,839		33,286
Segment assets at period end *	88,198	[1]	120,314	[1]	88,198	[1]	120,314	[1]
* Including unconsolidated investments		[1]		[1]		[1]		[1]

[1] Including unconsolidated investments