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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue	[1]									$ 746,044	$ 719,267	$ 692,812
Revenues		$ 192,442	$ 170,499	$ 184,065	$ 199,038	$ 190,465	$ 166,480	$ 178,299	$ 184,023
Cost of revenues		117,940	115,004	118,918	124,859	99,656	117,688	120,837	110,651	476,721	448,832	424,360
Gross profit		74,502	55,495	65,147	74,179	90,809	48,792	57,462	73,372	269,323	270,435	268,452
Research and development expenses		1,875	1,062	810	900	1,118	706	1,251	1,108	4,647	4,183	3,157
Selling and marketing expenses		4,123	3,783	3,276	3,865	3,813	8,578	3,712	3,699	15,047	19,802	15,600
General and administrative expenses		14,032	11,931	14,181	15,689	4,429	13,606	15,866	13,849	55,833	47,750	42,881
Impairment charge		0	0	0	0	13,464	0	0	0	0	13,464	0
Write-off of unsuccessful exploration activities		0	0	0	0	3	0	0	123	0	126	1,796
Operating income (loss)		54,472	38,719	46,880	53,725	67,982	25,902	36,633	54,593	193,796	185,110	205,018
Interest income		320	482	420	293	458	214	189	113	1,515	974	988
Interest expense, net		(17,568)	(20,076)	(21,517)	(21,223)	(22,034)	(18,700)	(15,846)	(14,344)	(80,384)	(70,924)	(54,142)
Derivatives and foreign currency transaction gains (losses)		(72)	205	19	472	(2,250)	(383)	(529)	(1,599)	624	(4,761)	2,654
Income attributable to sale of tax benefits		4,415	4,056	4,637	7,764	4,020	4,066	3,556	7,361	20,872	19,003	17,878
Other non-operating income (expense), net		(482)	244	1,027	91	117	309	7,373	(20)	880	7,779	(1,666)
Income from operations before income tax and equity in earnings (losses) of investees		41,085	23,630	31,466	41,122	48,293	11,408	31,376	46,104	137,303	137,181	170,730
Income tax (provision) benefit		(25,477)	(9,626)	3,529	(14,039)	(31,386)	(1,184)	(29,105)	26,942	(45,613)	(34,733)	(21,664)
Equity in earnings (losses) of investees, net		(1,481)	1,085	1,202	1,047	6,182	(117)	388	1,210	1,853	7,663	(1,957)
Net income		14,127	15,089	36,197	28,130	23,089	10,107	2,659	74,256
Net loss (income) attributable to noncontrolling interest		(1,521)	516	(2,259)	(2,184)	(4,869)	474	(3,002)	(4,748)	(5,448)	(12,145)	(14,695)
Net income (loss) attributable to the Company's stockholders		$ 12,606	$ 15,605	$ 33,938	$ 25,946	$ 18,220	$ 10,581	$ (343)	$ 69,508	$ 88,095	$ 97,966	$ 132,414
Net income (in dollars per share)		$ 0.25	$ 0.31	$ 0.67	$ 0.51	$ 0.36	$ 0.21	$ (0.01)	$ 1.37	$ 1.73	$ 1.93	$ 2.64
Net income (in dollars per share)		$ 0.24	$ 0.30	$ 0.66	$ 0.51	$ 0.36	$ 0.21	$ (0.01)	$ 1.36	$ 1.72	$ 1.92	$ 2.61
Basic (in shares)		51,017	50,933	50,800	50,709	50,691	50,645	50,623	50,614	50,867	50,643	50,110
Diluted (in shares)		51,511	51,334	51,094	51,012	50,936	50,963	50,958	51,051	51,227	50,969	50,769
Electricity [Member]
Revenue		$ 144,368	$ 123,978	$ 129,079	$ 142,908	$ 138,320	$ 116,891	$ 122,179	$ 132,489	$ 540,333	$ 509,879	$ 465,593
Cost of revenues		81,393	80,124	73,775	77,543	63,692	79,845	81,236	73,482	312,835	298,255	266,840
Product [Member]
Revenue		43,814	43,037	52,030	52,128	49,717	48,439	54,915	48,672	191,009	201,743	224,483
Cost of revenues		31,479	31,073	41,316	42,106	33,729	35,669	37,573	33,726	145,974	140,697	152,094
Energy Storage and Management Services [Member]
Revenue		4,260	3,484	2,956	4,002	2,428	1,150	1,205	2,862	14,702	7,645	2,736
Cost of revenues		$ 5,068	$ 3,807	$ 3,827	$ 5,210	$ 2,235	$ 2,174	$ 2,028	$ 3,443	$ 17,912	$ 9,880	$ 5,426

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