Condensed Consolidated Statements of Operations and Comprehensive Income (Unaudited) - USD ($) shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total consolidated revenues	$ 166,480,000	$ 157,185,000	$ 528,802,000	$ 526,447,000
Cost of revenues:
Cost of revenues	117,688,000	97,992,000	349,176,000	322,351,000
Gross profit	48,792,000	59,193,000	179,626,000	204,096,000
Operating expenses:
Research and development expenses	706,000	716,000	3,065,000	2,368,000
Selling and marketing expenses	8,578,000	3,630,000	15,989,000	12,083,000
General and administrative expenses	13,606,000	10,877,000	43,325,000	33,027,000
Write-off of unsuccessful exploration activities	0	0	119,000	0
Operating income	25,902,000	43,970,000	117,128,000	156,618,000
Other income (expense):
Interest income	214,000	255,000	516,000	861,000
Interest expense, net	(18,700,000)	(11,692,000)	(48,890,000)	(41,155,000)
Derivatives and foreign currency transaction gains (losses)	(383,000)	(1,001,000)	(2,511,000)	2,040,000
Income attributable to sale of tax benefits	4,066,000	3,506,000	14,983,000	14,019,000
Other non-operating income (expense), net	309,000	(1,592,000)	7,662,000	(1,678,000)
Income from continuing operations before income taxes and equity in earnings (losses) of investees	11,408,000	33,446,000	88,888,000	130,705,000
Income tax (provision) benefit	(1,184,000)	(6,224,000)	(3,347,000)	(49,993,000)
Equity in earnings (losses) of investees, net	(117,000)	337,000	1,481,000	(1,690,000)
Income from continuing operations	10,107,000	27,559,000	87,022,000	79,022,000
Net income attributable to noncontrolling interest	474,000	(3,599,000)	(7,276,000)	(11,228,000)
Net income attributable to the Company's stockholders	10,581,000	23,960,000	79,746,000	67,794,000
Comprehensive income:
Net income	10,107,000	27,559,000	87,022,000	79,022,000
Other comprehensive income (loss), net of related taxes:
Change in foreign currency translation adjustments	(91,000)	1,005,000	(1,059,000)	2,544,000
Change in unrealized gains or losses in respect of the Company's share in derivatives instruments of unconsolidated investment	1,012,000	618,000	4,175,000	271,000
Loss in respect of derivative instruments designated for cash flow hedge	20,000	20,000	60,000	113,000
Amortization of unrealized gains in respect of derivative instruments designated for cash flow hedge	(14,000)	(18,000)	(44,000)	(57,000)
Comprehensive income	11,034,000	29,184,000	90,154,000	81,893,000
Comprehensive income attributable to noncontrolling interest	458,000	(4,006,000)	(7,088,000)	(11,950,000)
Comprehensive income attributable to the Company's stockholders	$ 11,492,000	$ 25,178,000	$ 83,066,000	$ 69,943,000
Earnings per share attributable to the Company's stockholders:
Net income (in dollars per share)	$ 0.21	$ 0.48	$ 1.58	$ 1.36
Diluted:
Net income (in dollars per share)	$ 0.21	$ 0.47	$ 1.56	$ 1.34
Weighted average number of shares used in computation of earnings per share attributable to the Company's stockholders:
Basic (in shares)	50,645	50,367	50,627	49,942
Diluted (in shares)	50,963	50,867	50,985	50,669
Electricity [Member]
Revenues:
Revenues	$ 116,891,000	$ 110,876,000	$ 371,559,000	$ 337,548,000
Cost of revenues:
Cost of revenues	79,845,000	64,444,000	234,563,000	193,676,000
Product [Member]
Revenues:
Revenues	48,439,000	44,912,000	152,026,000	186,621,000
Cost of revenues:
Cost of revenues	35,669,000	32,218,000	106,968,000	125,102,000
Other Revenue [Member]
Revenues:
Revenues	1,150,000	1,397,000	5,217,000	2,278,000
Cost of revenues:
Cost of revenues	$ 2,174,000	$ 1,330,000	$ 7,645,000	$ 3,573,000