Condensed Consolidated Statements of Operations and Comprehensive Income (Unaudited) - USD ($) shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total consolidated revenues	$ 178,299,000	$ 179,364,000	$ 362,322,000	$ 369,262,000
Cost of revenues:
Cost of revenues	120,837,000	108,871,000	231,488,000	224,359,000
Gross profit	57,462,000	70,493,000	130,834,000	144,903,000
Operating expenses:
Research and development expenses	1,251,000	1,050,000	2,359,000	1,652,000
Selling and marketing expenses	3,712,000	4,090,000	7,411,000	8,453,000
General and administrative expenses	15,866,000	12,201,000	29,719,000	22,150,000
Write-off of unsuccessful exploration activities	0	0	119,000	0
Operating income	36,633,000	53,152,000	91,226,000	112,648,000
Other income (expense):
Interest income	189,000	362,000	302,000	606,000
Interest expense, net	(15,846,000)	(14,540,000)	(30,190,000)	(29,463,000)
Derivatives and foreign currency transaction gains (losses)	(529,000)	1,703,000	(2,128,000)	3,041,000
Income attributable to sale of tax benefits	3,556,000	4,356,000	10,917,000	10,513,000
Other non-operating income (expense), net	7,373,000	6,000	7,353,000	(86,000)
Income from continuing operations before income taxes and equity in losses of investees	31,376,000	45,039,000	77,480,000	97,259,000
Income tax (provision) benefit	(29,105,000)	(32,765,000)	(2,163,000)	(43,769,000)
Equity in earnings (losses) of investees, net	388,000	(428,000)	1,598,000	(2,027,000)
Income from continuing operations	2,659,000	11,846,000	76,915,000	51,463,000
Net income attributable to noncontrolling interest	(3,002,000)	(3,206,000)	(7,750,000)	(7,629,000)
Net income attributable to the Company's stockholders	(343,000)	8,640,000	69,165,000	43,834,000
Comprehensive income:
Net income	2,659,000	11,846,000	76,915,000	51,463,000
Other comprehensive income (loss), net of related taxes:
Change in foreign currency translation adjustments	(2,496,000)	1,461,000	(968,000)	1,539,000
Change in unrealized gains or losses in respect of the Company's share in derivatives instruments of unconsolidated investment	529,000	(916,000)	3,163,000	(347,000)
Loss in respect of derivative instruments designated for cash flow hedge	20,000	45,000	40,000	93,000
Amortization of unrealized gains in respect of derivative instruments designated for cash flow hedge	(15,000)	(15,000)	(30,000)	(39,000)
Comprehensive income	697,000	12,421,000	79,120,000	52,709,000
Comprehensive income attributable to noncontrolling interest	(2,428,000)	(3,613,000)	(7,546,000)	(8,025,000)
Comprehensive income attributable to the Company's stockholders	$ (1,731,000)	$ 8,808,000	$ 71,574,000	$ 44,684,000
Basic:
Net income (in dollars per share)	$ (0.01)	$ 0.17	$ 1.37	$ 0.88
Diluted:
Net income (in dollars per share)	$ (0.01)	$ 0.17	$ 1.36	$ 0.87
Weighted average number of shares used in computation of earnings per share attributable to the Company's stockholders:
Basic (in shares)	50,623	49,771	50,618	49,726
Diluted (in shares)	50,958	50,624	51,001	50,559
Dividend per share declared (in dollars per share)	$ 0.10	$ 0.08	$ 0.33	$ 0.25
Electricity [Member]
Revenues:
Revenues	$ 122,179,000	$ 110,896,000	$ 254,668,000	$ 226,672,000
Cost of revenues:
Cost of revenues	81,236,000	63,196,000	154,718,000	129,232,000
Product [Member]
Revenues:
Revenues	54,915,000	67,587,000	103,587,000	141,709,000
Cost of revenues:
Cost of revenues	37,573,000	43,432,000	71,299,000	92,884,000
Other Revenue [Member]
Revenues:
Revenues	1,205,000	881,000	4,067,000	881,000
Cost of revenues:
Cost of revenues	$ 2,028,000	$ 2,243,000	$ 5,471,000	$ 2,243,000