Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Net revenues	$ 189,898	$ 151,594
Operating income	59,496	50,543
Segment assets at period end	2,499,888	2,260,134	$ 2,461,569
Intersegment Eliminations [Member]
Net revenues	16,213	1,941
Electricity [Member]
Net revenues	115,776	107,868
Operating income	40,898	34,785
Segment assets at period end	2,282,837	2,048,471
Electricity [Member] | Intersegment Eliminations [Member]
Net revenues
Product Segment [Member]
Net revenues	74,122	43,726
Operating income	18,598	15,758
Segment assets at period end	217,051	211,663
Product Segment [Member] | Intersegment Eliminations [Member]
Net revenues	$ 16,213	$ 1,941