Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015		Dec. 31, 2015
Electricity [Member] | Intersegment Eliminations [Member]
Revenue
Electricity [Member]
Net revenue from external customers		104,001		90,926		211,869		180,879
Operating income		32,814		20,920		67,599		44,874
Segment assets at period end	[1]	2,031,650		2,080,209		2,031,650		2,080,209
Product Segment [Member] | Intersegment Eliminations [Member]
Revenue		19,266		10,900		21,206		30,657
Product Segment [Member]
Net revenue from external customers		55,860		49,561		99,586		79,839
Operating income		19,073		17,723		34,831		23,620
Segment assets at period end	[1]	241,363		196,341		241,363		196,341
Intersegment Eliminations [Member]
Revenue		19,266		10,900		21,206		30,657
Net revenue from external customers		159,861		140,487		311,455		260,718
Revenue		159,861		140,487		311,455		260,718
Operating income		51,887		38,643		102,430		68,494
Segment assets at period end		$ 2,273,013	[1]	$ 2,276,550	[1]	$ 2,273,013	[1]	$ 2,276,550	[1]	$ 2,273,982

[1] Including unconsolidated investments