Condensed Consolidated Statements of Operations and Comprehensive Income (Unaudited) - USD ($) shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues:
Electricity	$ 111,777,000	$ 104,001,000	$ 227,553,000	$ 211,869,000
Product	67,587,000	55,860,000	141,709,000	99,586,000
Total revenues	179,364,000	159,861,000	369,262,000	311,455,000
Cost of revenues:
Electricity	65,439,000	62,243,000	131,475,000	125,929,000
Product	43,432,000	31,822,000	92,884,000	55,857,000
Total cost of revenues	108,871,000	94,065,000	224,359,000	181,786,000
Gross profit	70,493,000	65,796,000	144,903,000	129,669,000
Operating expenses:
Research and development expenses	1,050,000	595,000	1,652,000	944,000
Selling and marketing expenses	4,090,000	3,668,000	8,453,000	7,343,000
General and administrative expenses	12,201,000	8,783,000	22,150,000	17,532,000
Write-off of unsuccessful exploration activities	0	863,000	0	1,420,000
Operating income	53,152,000	51,887,000	112,648,000	102,430,000
Other income (expense):
Interest income	362,000	245,000	606,000	565,000
Interest expense, net	(14,540,000)	(18,401,000)	(29,463,000)	(34,424,000)
Derivatives and foreign currency transaction gains (losses)	1,703,000	(4,332,000)	3,041,000	(2,370,000)
Income attributable to sale of tax benefits	4,356,000	4,519,000	10,513,000	8,917,000
Other non-operating income (expense), net	6,000	49,000	(86,000)	240,000
Income from continuing operations before income taxes and equity in losses of investees	45,039,000	33,967,000	97,259,000	75,358,000
Income tax (provision) benefit	(6,369,000)	(7,890,000)	(17,255,000)	(17,399,000)
Equity in losses of investees, net	(428,000)	(1,144,000)	(2,027,000)	(2,081,000)
Income from continuing operations	38,242,000	24,933,000	77,977,000	55,878,000
Net income attributable to noncontrolling interest	(3,206,000)	(584,000)	(7,629,000)	(2,258,000)
Net income attributable to the Company's stockholders	35,036,000	24,349,000	70,348,000	53,620,000
Comprehensive income:
Net income	38,242,000	24,933,000	77,977,000	55,878,000
Other comprehensive income (loss), net of related taxes:
Change in foreign currency translation adjustments	1,461,000		1,539,000
Change in unrealized gains or losses in respect of the Company's share in derivatives instruments of unconsolidated investment	(916,000)	(1,987,000)	(347,000)	(5,166,000)
Loss in respect of derivative instruments designated for cash flow hedge	20,000	22,000	42,000	43,000
Amortization of unrealized gains in respect of derivative instruments designated for cash flow hedge	(15,000)	(24,000)	(39,000)	(48,000)
Comprehensive income	38,792,000	22,944,000	79,172,000	50,707,000
Comprehensive income attributable to noncontrolling interest	(3,613,000)	(584,000)	(8,025,000)	(2,258,000)
Comprehensive income attributable to the Company's stockholders	$ 35,179,000	$ 22,360,000	$ 71,147,000	$ 48,449,000
Basic:
Net income (in dollars per share)	$ 0.70	$ 0.49	$ 1.41	$ 1.09
Diluted:
Net income (in dollars per share)	$ 0.69	$ 0.49	$ 1.39	$ 1.07
Weighted average number of shares used in computation of earnings per share attributable to the Company's stockholders:
Basic (in shares)	49,771	49,456	49,726	49,314
Diluted (in shares)	50,624	50,137	50,559	49,977
Dividend per share declared (in dollars per share)	$ 0.08	$ 0.07	$ 0.25	$ 0.38