Note 23 - Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Revenue									$ 719,267		$ 692,812		$ 662,591
Revenues	$ 190,465	$ 166,480	$ 178,299	$ 184,023	$ 166,365	$ 157,185	$ 179,364	$ 189,898	719,267	[1]	692,812	[1]	662,591	[1]
Cost of revenues	99,656	117,688	120,837	110,651	102,009	97,992	108,871	115,488	448,832		424,360		391,796
Gross profit	90,809	48,792	57,462	73,372	64,356	59,193	70,493	74,410	270,435		268,452		270,795
Research and development expenses	1,118	706	1,251	1,108	789	716	1,050	602	4,183		3,157		2,762
Selling and marketing expenses	3,813	8,578	3,712	3,699	3,517	3,630	4,090	4,363	19,802		15,600		16,424
General and administrative expenses	4,429	13,606	15,866	13,849	9,854	10,877	12,201	9,949	47,750		42,881		46,710
Impairment charge	13,464								13,464
Write-off of unsuccessful exploration activities	3			123	1,796				126		1,796		3,017
Operating income (loss)	67,982	25,902	36,633	54,593	48,400	43,970	53,152	59,496	185,110		205,018		201,882
Interest income	458	214	189	113	127	255	362	244	974		988		971
Interest expense, net	(22,034)	(18,700)	(15,846)	(14,344)	(12,987)	(11,692)	(14,540)	(14,923)	(70,924)		(54,142)		(67,389)
Derivatives and foreign currency transaction gains (losses)	(2,250)	(383)	(529)	(1,599)	614	(1,001)	1,703	1,338	(4,761)		2,654		(5,534)
Income attributable to sale of tax benefits	4,020	4,066	3,556	7,361	3,859	3,506	4,356	6,157	19,003		17,878		16,503
Other non-operating income (expense), net	117	309	7,373	(20)	12	(1,592)	6	(92)	7,779		(1,666)		(5,345)
Income from continuing operations, before income tax and equity in earnings (losses) of investees	48,293	11,408	31,376	46,104	40,025	33,446	45,039	52,220	137,181		170,730		141,088
Income tax (provision) benefit	(31,386)	(1,184)	(29,105)	26,942	28,329	(6,224)	(32,765)	(11,004)	(34,733)		(21,664)		(37,059)
Equity in earnings (losses) of investees, net	6,182	(117)	388	1,210	(267)	337	(428)	(1,599)	7,663		(1,957)		(7,735)
Income from continuing operations	23,089	10,107	2,659	74,256	68,087	27,559	11,846	39,617
Net loss (income) attributable to noncontrolling interest	(4,869)	474	(3,002)	(4,748)	(3,467)	(3,599)	(3,206)	(4,423)	12,145		14,695		7,586
Net income attributable to the Company’s stockholders	$ 18,220	$ 10,581	$ (343)	$ 69,508	$ 64,620	$ 23,960	$ 8,640	$ 35,194	$ 97,966		$ 132,414		$ 88,708
Net income (in dollars per share)	$ 0.36	$ 0.21	$ (0.01)	$ 1.37	$ 1.28	$ 0.48	$ 0.17	$ 0.71	$ 1.93		$ 2.64		$ 1.79
Net income (in dollars per share)	$ 0.36	$ 0.21	$ (0.01)	$ 1.36	$ 1.27	$ 0.47	$ 0.17	$ 0.70	$ 1.92		$ 2.61		$ 1.77
Basic (in shares)	50,691	50,645	50,623	50,614	50,607	50,367	49,771	49,680	50,643		50,110		49,469
Diluted (in shares)	50,936	50,963	50,958	51,051	51,053	50,867	50,624	50,491	50,969		50,769		50,140
Electricity [Member]
Revenue	$ 138,320	$ 116,891	$ 122,179	$ 132,489	$ 128,045	$ 110,876	$ 110,896	$ 115,776	$ 509,879		$ 465,593		$ 436,292
Cost of revenues	63,692	79,845	81,236	73,482	73,164	64,444	63,196	66,036	298,255		266,840		261,573
Product [Member]
Revenue	49,717	48,439	54,915	48,672	37,862	44,912	67,587	74,122	201,743		224,483		226,299
Cost of revenues	33,729	35,669	37,573	33,726	26,992	32,218	43,432	49,452	$ 140,697		$ 152,094		$ 130,223
Product and Service, Other [Member]
Revenue	2,428	1,150	1,205	2,862	458	1,397	881
Cost of revenues	$ 2,235	$ 2,174	$ 2,028	$ 3,443	$ 1,853	$ 1,330	$ 2,243

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