Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016	Dec. 31, 2016
Operating income	$ 53,152		$ 51,887	$ 112,648		$ 102,430
Segment assets at period end	2,510,633	[1]	2,273,013	2,510,633	[1]	2,273,013	$ 2,461,569
Net revenues	179,364		159,861	369,262		311,455
Intersegment Eliminations [Member]
Net revenues	16,565		19,266	32,778		21,206
Electricity [Member]
Operating income	36,117		32,814	77,015		67,599
Segment assets at period end	2,323,618	[1]	2,031,650	2,323,618	[1]	2,031,650
Net revenues	111,777		104,001	227,553		211,869
Electricity [Member] | Intersegment Eliminations [Member]
Net revenues
Product Segment [Member]
Operating income	17,035		19,073	35,633		34,831
Segment assets at period end	187,015	[1]	241,363	187,015	[1]	241,363
Net revenues	67,587		55,860	141,709		99,586
Product Segment [Member] | Intersegment Eliminations [Member]
Net revenues	$ 16,565		$ 19,266	$ 32,778		$ 21,206

[1] Electricity segment assets include goodwill in the amount of $20.1 million