Consolidated Statements of Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Parent [Member]	Noncontrolling Interest [Member]	Total
Balance (in shares) at Dec. 31, 2016	49,667
Balance at Dec. 31, 2016	$ 50	$ 869,463	$ 215,352	$ (8,175)	$ 1,076,690	$ 91,582	$ 1,168,272
Stock-based compensation		1,713			1,713		1,713
Exercise of options by employees and directors (in shares)	39
Exercise of options by employees and directors
Cash paid to noncontrolling interest						(6,807)	(6,807)
Cash dividend declared			(8,448)		(8,448)		(8,448)
Net income			35,194		35,194	4,079	39,273
Other comprehensive income (loss), net of related taxes:
Currency translation adjustment				89	89	(11)
Loss in respect of derivative instruments designated for cash flow hedge				48	48		48
Change in unrealized gains or losses in respect of the Company's share in derivative instruments of unconsolidated investment				569	569		569
Amortization of unrealized gains in respect of derivative instruments designated for cash flow hedge				(24)	(24)		(24)
Balance (in shares) at Dec. 31, 2016	49,667
Balance at Dec. 31, 2016	$ 50	869,463	215,352	(8,175)	1,076,690	91,582	1,168,272
Other comprehensive income (loss), net of related taxes:
Stock-based compensation		1,713			1,713		1,713
Exercise of options by employees and directors (in shares)	39
Exercise of options by employees and directors
Cash paid to noncontrolling interest						(6,807)	(6,807)
Cash dividend declared			(8,448)		(8,448)		(8,448)
Net income			35,194		35,194	4,079	39,273
Balance (in shares) at Mar. 31, 2017	49,706
Balance at Mar. 31, 2017	$ 50	871,176	242,098	(7,493)	1,105,831	88,843	1,194,674
Balance (in shares) at Dec. 31, 2017	50,609
Balance at Dec. 31, 2017	$ 51	888,778	327,255	(4,706)	1,211,378	84,322	1,295,700
Stock-based compensation		1,707			1,707		1,707
Exercise of options by employees and directors (in shares)	8
Exercise of options by employees and directors
Cash paid to noncontrolling interest						(4,674)	(4,674)
Cash dividend declared			(11,640)		(11,640)		(11,640)
Net income			69,508		69,508	4,482	73,990
Other comprehensive income (loss), net of related taxes:
Currency translation adjustment				1,158	1,158	370	1,528
Loss in respect of derivative instruments designated for cash flow hedge				20	20		20
Change in unrealized gains or losses in respect of the Company's share in derivative instruments of unconsolidated investment				2,634	2,634		2,634
Amortization of unrealized gains in respect of derivative instruments designated for cash flow hedge				(15)	(15)		(15)
Balance (in shares) at Dec. 31, 2017	50,609
Balance at Dec. 31, 2017	$ 51	888,778	327,255	(4,706)	1,211,378	84,322	1,295,700
Other comprehensive income (loss), net of related taxes:
Stock-based compensation		1,707			1,707		1,707
Exercise of options by employees and directors (in shares)	8
Exercise of options by employees and directors
Cash paid to noncontrolling interest						(4,674)	(4,674)
Cash dividend declared			(11,640)		(11,640)		(11,640)
Net income			69,508		69,508	4,482	73,990
Balance (in shares) at Mar. 31, 2018	50,617
Balance at Mar. 31, 2018	$ 51	890,485	410,758	(909)	1,300,385	84,500	1,384,885
Other comprehensive income (loss), net of related taxes:
Cumulative effect of changes in accounting principles			$ 25,635		$ 25,635		$ 25,635