Note 21 - Business Segments (Details) - Summarized Financial Information Concerning Reportable Segments - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net revenues from external customers		$ 171,074	$ 162,852	$ 140,487	$ 120,231	$ 149,243	$ 140,242	$ 127,603	$ 142,436	$ 594,644	$ 559,524	$ 533,239
Intersegment revenues	[1]									594,644	559,524	533,239
Depreciation and amortization expense										107,206	100,799	92,932
Operating income		49,088	$ 46,479	$ 38,643	$ 29,851	34,801	$ 43,788	$ 22,330	$ 42,571	164,061	143,490	96,958
Segment assets at period end *	[2]	2,293,044				2,121,556				2,293,044	2,121,556	2,159,433
Expenditures for long-lived assets										152,450	158,781	204,628
Including unconsolidated investments		0								0		7,076
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues										48,559	44,718	37,248
Electricity Member
Segment Reporting Information [Line Items]
Net revenues from external customers										375,920	382,301	329,747
Depreciation and amortization expense										103,892	97,826	88,853
Operating income										99,345	90,401	54,265
Segment assets at period end *	[2]	2,044,346				1,963,486				2,044,346	1,963,486	2,017,838
Expenditures for long-lived assets										149,666	155,323	203,047
Including unconsolidated investments		0								0		7,076
Product Segment [Member]
Segment Reporting Information [Line Items]
Net revenues from external customers										218,724	177,223	203,492
Depreciation and amortization expense										3,314	2,973	4,079
Operating income										64,716	53,089	42,693
Segment assets at period end *	[2]	248,698				$ 158,070				248,698	158,070	141,595
Expenditures for long-lived assets										2,784	3,458	1,581
Including unconsolidated investments		$ 0								0
Product Segment [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment revenues										$ 48,559	$ 44,718	$ 37,248

[1] Revenues as reported in the geographic area in which they originate.

[2] Including unconsolidated investments