Note 24 - 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, 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
Revenues:
Electricity	$ 114,628	$ 109,795	$ 104,001	$ 107,868	$ 97,796	$ 97,245	$ 90,926	$ 89,953	$ 436,292	$ 375,920	$ 382,301
Product	51,891	74,822	55,860	43,726	73,278	65,607	49,561	30,278	226,299	218,724	177,223
Total revenues	166,519	184,617	159,861	151,594	171,074	162,852	140,487	120,231	662,591	594,644	559,524
Cost of revenues:
Electricity	69,163	66,481	62,243	63,686	63,008	61,501	62,522	55,581	261,573	242,612	246,630
Product	30,719	43,647	31,822	24,035	43,927	42,019	27,182	20,625	130,223	133,753	109,143
Total cost of revenues	99,882	110,128	94,065	87,721	106,935	103,520	89,704	76,206	391,796	376,365	355,773
Gross profit	66,637	74,489	65,796	63,873	64,139	59,332	50,783	44,025	270,795	218,279	203,751
Operating expenses:
Research and development expenses	732	1,086	595	349	668	335	414	363	2,762	1,780	783
Selling and marketing expenses	4,288	4,793	3,668	3,675	3,978	4,383	4,283	3,433	16,424	16,077	15,425
General and administrative expenses	10,085	19,093	8,783	8,749	9,185	7,950	7,443	10,204	46,710	34,782	28,614
Write-off of unsuccessful exploration activities	303	1,294	863	557	1,220	185		174	3,017	1,579	15,439
Operating income	51,229	48,223	51,887	50,543	49,088	46,479	38,643	29,851	201,882	164,061	143,490
Interest income	140	266	245	320	191	53	44	9	971	297	312
Interest expense, net	(15,828)	(17,137)	(18,401)	(16,023)	(18,142)	(17,748)	(18,859)	(17,828)	(67,389)	(72,577)	(84,654)
Foreign currency translation and transaction gains (losses)	(2,942)	(222)	(4,332)	1,962	(981)	1,296	(571)	(1,366)	(5,534)	(1,622)	(5,839)
Income attributable to sale of tax benefits	4,123	3,463	4,519	4,398	6,514	8,634	4,731	5,552	16,503	25,431	24,143
Other non-operating income (expense), net	(39)	(5,546)	49	191	(468)	(131)	(1,675)	283	(5,345)	(1,991)	756
Income (loss) from continuing operations, before income taxes and equity in income of investees	36,683	29,047	33,967	41,391	36,202	38,583	22,313	16,501	141,088	113,599	85,836
Income tax benefit (provision)	(2,450)	(11,988)	(7,890)	(9,509)	(11,438)	38,211	(6,056)	(5,459)	(31,837)	15,258	(27,608)
Equity in income (losses) of investees	(3,001)	(2,653)	(1,144)	(937)	(616)	(3,133)	(984)	(775)	(7,735)	(5,508)	(3,213)
Net income (loss)	31,232	14,406	24,933	30,945	24,148	73,661	15,273	10,267	101,516	123,349	55,015
Net loss (income) attributable to noncontrolling interest	(3,002)	(2,326)	(584)	(1,674)	(1,160)	(1,522)	(859)	(235)	7,586	3,776	833
Net income (loss) attributable to the Company's stockholders	$ 28,230	$ 12,080	$ 24,349	$ 29,271	$ 22,988	$ 72,139	$ 14,414	$ 10,032	$ 93,930	$ 119,573	$ 54,182
Net income (in dollars per share)	$ 0.57	$ 0.24	$ 0.49	$ 0.60	$ 0.47	$ 1.47	$ 0.29	$ 0.21	$ 1.90	$ 2.46	$ 1.19
Net income (in dollars per share)	$ 0.56	$ 0.24	$ 0.49	$ 0.59	$ 0.46	$ 1.41	$ 0.28	$ 0.21	$ 1.87	$ 2.43	$ 1.18
Basic (in shares)	49,647	49,599	49,456	49,173	49,074	49,023	48,881	47,244	49,469	48,562	45,508
Diluted (in shares)	50,293	50,289	50,137	49,782	49,668	51,113	50,600	48,079	50,140	49,187	45,859