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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Revenues:
Revenues	$ 166,365	$ 157,185	$ 179,364	$ 189,898	$ 166,519	$ 184,617	$ 159,861	$ 151,594	$ 692,812	[1]	$ 662,591	[1]	$ 594,644	[1]
Cost of revenues:
Cost of revenues	102,009	97,992	108,871	115,488	99,882	110,128	94,065	87,721	424,360		391,796		376,365
Gross margin	64,356	59,193	70,493	74,410	66,637	74,489	65,796	63,873	268,452		270,795		218,279
Operating expenses:
Research and development expenses	789	716	1,050	602	732	1,086	595	349	3,157		2,762		1,780
Selling and marketing expenses	3,517	3,630	4,090	4,363	4,288	4,793	3,668	3,675	15,600		16,424		16,077
General and administrative expenses	9,854	10,877	12,201	9,949	10,085	19,093	8,783	8,749	42,881		46,710		34,782
Write-off of unsuccessful exploration activities	1,796				303	1,294	863	557	1,796		3,017		1,579
Operating income	48,400	43,970	53,152	59,496	51,229	48,223	51,887	50,543	205,018		201,882		164,061
Interest income	127	255	362	244	140	266	245	320	988		971		297
Interest expense, net	(12,987)	(11,692)	(14,540)	(14,923)	(15,828)	(17,137)	(18,401)	(16,023)	(54,142)		(67,389)		(72,577)
Derivatives and foreign currency transaction gains (losses)	614	(1,001)	1,703	1,338	(2,942)	(222)	(4,332)	1,962	2,654		(5,534)		(1,622)
Income attributable to sale of tax benefits	3,859	3,506	4,356	6,157	4,123	3,463	4,519	4,398	17,878		16,503		25,431
Other non-operating income (expense), net	12	(1,592)	6	(92)	(39)	(5,546)	49	191	(1,666)		(5,345)		(1,991)
Income (loss) from continuing operations, before income taxes and equity in income (losses) of investees	40,025	33,446	45,039	52,220	36,683	29,047	33,967	41,391	170,730		141,088		113,599
Income tax (provision) benefit	28,329	(6,224)	(32,765)	(11,004)	(7,356)	(12,109)	(8,515)	(9,079)	(21,664)		(37,059)		16,057
Equity in income (losses) of investees	(267)	337	(428)	(1,599)	(3,001)	(2,653)	(1,144)	(937)	(1,957)		(7,735)		(5,508)
Net income (loss)	68,087	27,559	11,846	39,617	26,326	14,285	24,308	31,375	147,109		96,294		124,148
Net loss (income) attributable to noncontrolling interest	(3,467)	(3,599)	(3,206)	(4,423)	(3,002)	(2,326)	(584)	(1,674)	14,695		7,586		3,776
Net income (loss) attributable to the Company's stockholders	$ 64,620	$ 23,960	$ 8,640	$ 35,194	$ 23,324	$ 11,959	$ 23,724	$ 29,701	$ 132,414		$ 88,708		$ 120,372
Net income (in dollars per share)	$ 1.28	$ 0.48	$ 0.17	$ 0.71	$ 0.47	$ 0.24	$ 0.48	$ 0.60	$ 2.64		$ 1.79		$ 2.48
Net income (in dollars per share)	$ 1.27	$ 0.47	$ 0.17	$ 0.70	$ 0.46	$ 0.24	$ 0.47	$ 0.60	$ 2.61		$ 1.77		$ 2.45
Basic (in shares)	50,607	50,367	49,771	49,680	49,647	49,599	49,456	49,173	50,110		49,469		48,562
Diluted (in shares)	51,053	50,867	50,624	50,491	50,293	50,289	50,137	49,782	50,769		50,140		49,187
Electricity [Member]
Revenues:
Revenues	$ 128,503	$ 112,273	$ 111,777	$ 115,776	$ 114,628	$ 109,795	$ 104,001	$ 107,868	$ 468,329		$ 436,292		$ 375,920
Cost of revenues:
Cost of revenues	75,017	65,774	65,439	66,036	69,163	66,481	62,243	63,686	272,266		261,573		242,612
Product [Member]
Revenues:
Revenues	37,862	44,912	67,587	74,122	51,891	74,822	55,860	43,726	224,483		226,299		218,724
Cost of revenues:
Cost of revenues	$ 26,992	$ 32,218	$ 43,432	$ 49,452	$ 30,719	$ 43,647	$ 31,822	$ 24,035	$ 152,094		$ 130,223		$ 133,753

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