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, 2013		Jun. 30, 2012		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2012		Mar. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 152,679		$ 126,708		$ 126,708		$ 271,585		$ 271,585		$ 256,156		$ 256,156
Operating income	37,862		24,383		24,383		45,546		45,546		49,048		49,048
Segment assets at period end *	2,137,900	[1]	2,294,100	[1]	2,294,100	[1]	2,137,900	[1]	2,137,900	[1]	2,294,100	[1]	2,294,100	[1]	2,133,925	2,087,523
* Including unconsolidated investments	3,524		3,783		3,783		3,524		3,524		3,783		3,783			2,591
Intersegment Eliminations [Member] | Product Segment [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	18,821		8,941				25,402				21,907
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues	18,821		8,941				25,402				21,907
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers	87,713		81,882				156,011				161,225
Operating income	22,639		16,567				21,325				31,365
Segment assets at period end *	2,039,219	[1]	2,204,421	[1]	2,204,421	[1]	2,039,219	[1]	2,039,219	[1]	2,204,421	[1]	2,204,421	[1]
* Including unconsolidated investments	3,524		2,438		2,438		3,524		3,524		2,438		2,438
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers	64,966		44,826				115,574				94,931
Operating income	15,223		7,816				24,221				17,683
Segment assets at period end *	98,681	[1]	89,679	[1]	89,679	[1]	98,681	[1]	98,681	[1]	89,679	[1]	89,679	[1]
* Including unconsolidated investments			$ 1,345		$ 1,345						$ 1,345		$ 1,345

[1] Including unconsolidated investments