Condensed Consolidated Statements of Operations and Comprehensive Income (Unaudited) - USD ($) shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues:
Electricity	$ 112,273,000	$ 109,795,000	$ 339,826,000	$ 321,664,000
Product	44,912,000	74,822,000	186,621,000	174,408,000
Total revenues	157,185,000	184,617,000	526,447,000	496,072,000
Cost of revenues:
Electricity	65,774,000	66,481,000	197,249,000	192,410,000
Product	32,218,000	43,647,000	125,102,000	99,504,000
Total cost of revenues	97,992,000	110,128,000	322,351,000	291,914,000
Gross profit	59,193,000	74,489,000	204,096,000	204,158,000
Operating expenses:
Research and development expenses	716,000	1,086,000	2,368,000	2,030,000
Selling and marketing expenses	3,630,000	4,793,000	12,083,000	12,136,000
General and administrative expenses	10,877,000	19,093,000	33,027,000	36,625,000
Write-off of unsuccessful exploration activities	0	1,294,000	0	2,714,000
Operating income	43,970,000	48,223,000	156,618,000	150,653,000
Other income (expense):
Interest income	255,000	266,000	861,000	831,000
Interest expense, net	(11,692,000)	(17,137,000)	(41,155,000)	(51,561,000)
Derivatives and foreign currency transaction gains (losses)	(1,001,000)	(222,000)	2,040,000	(2,592,000)
Income attributable to sale of tax benefits	3,506,000	3,463,000	14,019,000	12,380,000
Other non-operating expense, net	(1,592,000)	(5,546,000)	(1,678,000)	(5,306,000)
Income from continuing operations before income taxes and equity in losses of investees	33,446,000	29,047,000	130,705,000	104,405,000
Income tax provision	(11,003,000)	(11,988,000)	(28,258,000)	(29,387,000)
Equity in earnings (losses) of investees, net	337,000	(2,653,000)	(1,690,000)	(4,734,000)
Income from continuing operations	22,780,000	14,406,000	100,757,000	70,284,000
Net income attributable to noncontrolling interest	(3,599,000)	(2,326,000)	(11,228,000)	(4,584,000)
Net income attributable to the Company's stockholders	19,181,000	12,080,000	89,529,000	65,700,000
Comprehensive income:
Net income	22,780,000	14,406,000	100,757,000	70,284,000
Other comprehensive income (loss), net of related taxes:
Change in foreign currency translation adjustments	1,005,000		2,544,000
Change in unrealized gains or losses in respect of the Company's share in derivatives instruments of unconsolidated investment	618,000	1,337,000	271,000	(3,829,000)
Loss in respect of derivative instruments designated for cash flow hedge	20,000	22,000	62,000	65,000
Amortization of unrealized gains in respect of derivative instruments designated for cash flow hedge	(18,000)	(24,000)	(57,000)	(72,000)
Comprehensive income	24,405,000	15,741,000	103,577,000	66,448,000
Comprehensive income attributable to noncontrolling interest	(4,006,000)	(2,326,000)	(11,950,000)	(4,584,000)
Comprehensive income attributable to the Company's stockholders	$ 20,399,000	$ 13,415,000	$ 91,627,000	$ 61,864,000
Basic:
Net income (in dollars per share)	$ 0.38	$ 0.24	$ 1.79	$ 1.33
Diluted:
Net income (in dollars per share)	$ 0.38	$ 0.24	$ 1.77	$ 1.31
Weighted average number of shares used in computation of earnings per share attributable to the Company's stockholders:
Basic (in shares)	50,367	49,599	49,942	49,410
Diluted (in shares)	50,867	50,289	50,669	50,097
Dividend per share declared (in dollars per share)	$ 0.08	$ 0.07	$ 0.33	$ 0.45