Note 20 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net revenues from external customers		$ 166,519	$ 184,617	$ 159,861	$ 151,594	$ 171,074	$ 162,852	$ 140,487	$ 120,231	$ 662,591	$ 594,644	$ 559,524
Depreciation and amortization expense										105,977	107,206	100,799
Operating income		51,229	$ 48,223	$ 51,887	$ 50,543	49,088	$ 46,479	$ 38,643	$ 29,851	201,882	164,061	143,490
Segment assets at period end	[1]	2,461,569				2,273,982				2,461,569	2,273,982	2,121,556
Expenditures for long-lived assets										151,930	152,450	158,781
Intersegment Eliminations [Member]
Net revenues from external customers										56,075	48,559	44,718
Electricity [Member]
Net revenues from external customers										436,292	375,920	382,301
Depreciation and amortization expense										102,698	103,892	97,826
Operating income										126,828	99,345	90,401
Segment assets at period end	[1]	2,204,444				2,044,346				2,204,444	2,044,346	1,963,486
Expenditures for long-lived assets										147,211	149,666	155,323
Electricity [Member] | Intersegment Eliminations [Member]
Net revenues from external customers
Product Segment [Member]
Net revenues from external customers										226,299	218,724	177,223
Depreciation and amortization expense										3,279	3,314	2,973
Operating income										75,054	64,716	53,089
Segment assets at period end	[1]	$ 257,125				$ 229,636				257,125	229,636	158,070
Expenditures for long-lived assets										4,719	2,784	3,458
Product Segment [Member] | Intersegment Eliminations [Member]
Net revenues from external customers										$ 56,075	$ 48,559	$ 44,718

[1] Including unconsolidated investments