Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Noncontrolling Interests' Equity [Member]
Beginning Balance at Dec. 31, 2014	$ 536,944	$ 25,640	$ 139,573	$ (66,262)	$ (83,945)	$ 520,540	$ 1,398
Issuance of common stock under stock option plan	777	45	732
Purchase of common stock	(2,000)			(2,000)
Stock-based and deferred compensation	3,694	24	3,854	(184)
Net income	76,037					75,968	69
Other comprehensive income	(41,210)				(41,143)		(67)
Cash dividends paid:
Common stock	(16,300)					(16,300)
Non-qualified stock option and stock award income tax benefit	442		442
Ending Balance at Dec. 31, 2015	558,384	25,709	144,601	(68,446)	(125,088)	580,208	1,400
Issuance of common stock under stock option plan	4,017	168	3,849
Purchase of common stock	(2,408)			(2,408)
Stock-based and deferred compensation	9,526	18	9,592	(84)
Net income	86,198					86,191	7
Other comprehensive income	(2,472)				(2,377)		(95)
Cash dividends paid:
Common stock	(17,329)					(17,329)
Ending Balance at Dec. 31, 2016	635,916	25,895	158,042	(70,938)	(127,465)	649,070	1,312
Issuance of common stock under stock option plan	3,370	104	3,266
Purchase of common stock	(6,000)			(6,000)
Stock-based and deferred compensation	7,549	72	9,100	(1,623)
Net income	91,547					91,578	(31)
Other comprehensive income	27,981				27,902		79
Cash dividends paid:
Common stock	(18,907)					(18,907)
Payment of cash dividends to noncontrolling interest	(542)						(542)
Ending Balance at Dec. 31, 2017	$ 740,914	$ 26,071	$ 170,408	$ (78,561)	$ (99,563)	$ 721,741	$ 818