Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net income	$ 39,735,000	$ 30,945,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	27,059,000	26,181,000
Amortization of premium from senior unsecured bonds		(77,000)
Accretion of asset retirement obligation	455,000	406,000
Stock-based compensation	1,713,000	842,000
Amortization of deferred lease income	(671,000)	(671,000)
Income attributable to sale of tax benefits, net of interest expense	(4,335,000)	(3,475,000)
Equity in losses of investees	1,600,000	937,000
Mark-to-market of derivative instruments	(1,519,000)	(162,000)
Write-off of unsuccessful exploration activities	0	557,000
Gain on severance pay fund asset	(947,000)	(564,000)
Deferred income tax provision	6,612,000	6,643,000
Liability for unrecognized tax benefits	574,000	254,000
Deferred lease revenues	(92,000)	88,000
Changes in operating assets and liabilities, net of amounts acquired:
Receivables	19,092,000	(21,925,000)
Costs and estimated earnings in excess of billings on uncompleted contracts	(4,352,000)	(4,777,000)
Inventories	(5,800,000)	1,279,000
Prepaid expenses and other	6,873,000	(1,808,000)
Deposits and other	(557,000)	80,000
Accounts payable and accrued expenses	681,000	(4,846,000)
Billings in excess of costs and estimated earnings on uncompleted contracts	14,035,000	2,975,000
Liabilities for severance pay	930,000	(205,000)
Other long-term liabilities	(1,553,000)	317,000
Net cash provided by operating activities	71,463,000	27,044,000
Cash flows from investing activities:
Net change in restricted cash, cash equivalents and marketable securities	(25,617,000)	(14,626,000)
Capital expenditures	(52,885,000)	(31,031,000)
Investment in unconsolidated companies	(14,918,000)
Cash paid for acquisition of controlling interest in a subsidiary, net of cash acquired	(35,300,000)
Decrease (increase) in severance pay fund asset, net of payments made to retired employees	(18,000)	1,037,000
Net cash used in investing activities	(128,738,000)	(44,620,000)
Cash flows from financing activities:
Proceeds from exercise of options by employees		4,195,000
Proceeds from revolving credit lines with banks	50,000,000	49,700,000
Repayment of revolving credit lines with banks	(20,000,000)	(40,700,000)
Cash received from noncontrolling interest	1,411,000	1,972,000
Repayments of long-term debt	(13,405,000)	(11,631,000)
Cash paid to noncontrolling interest	(6,807,000)	(6,317,000)
Payments of capital leases	(408,000)
Deferred debt issuance costs	(1,144,000)	(1,592,000)
Cash dividends paid	(8,448,000)	(15,472,000)
Net cash provided by (used in) financing activities	1,199,000	(19,845,000)
Net change in cash and cash equivalents	(56,076,000)	(37,421,000)
Cash and cash equivalents at beginning of period	230,214,000	185,919,000
Cash and cash equivalents at end of period	174,138,000	148,498,000
Supplemental non-cash investing and financing activities:
Increase (decrease) in accounts payable related to purchases of property, plant and equipment	1,801,000	(5,296,000)
Accrued liabilities related to financing activities		$ 3,768,000