Consolidated Statements Of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss)	Treasury shares, at cost
Beginning Balance at Dec. 31, 2014	$ 1,133,202	$ 3,024	$ 86,388	$ 1,030,387	$ 71,959	$ (58,556)
Beginning Balance, in shares, at Dec. 31, 2014		(30,235,164)				(1,428,495)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under stock-based plans	(1,604)					$ 4,292
Shares issued under stock-based plans, in shares						112,757
Adjustments to APIC, stock-based plans			(5,896)
Stock-based compensation expense	3,247		3,247
Income tax benefit from stock-based compensation	256		256
Net Income	9,916			9,916
Dividends paid	(2,798)			(2,798)
Marketable securities unrealized gain (loss)	(14,310)				(14,310)
Accelerated share repurchases			3,293
Purchases of treasury shares, in shares						(2,363,198)
Purchases of treasury shares	(60,975)					$ (57,682)
Balance at Jun. 30, 2015	1,066,934	$ 3,024	80,702	1,037,505	57,649	$ (111,946)
Balance, in shares, at Jun. 30, 2015		(30,235,164)				(3,678,936)
Beginning Balance at Dec. 31, 2015	998,159	$ 3,024	86,557	1,052,567	14,842	$ (158,831)
Beginning Balance, in shares, at Dec. 31, 2015		(30,235,164)				(5,362,684)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under stock-based plans	(286)					$ 6,294
Shares issued under stock-based plans, in shares						211,861
Adjustments to APIC, stock-based plans			(6,580)
Stock-based compensation expense	2,859		2,859
Net Income	12,731			12,731
Dividends paid	(3,907)			(3,907)
Marketable securities unrealized gain (loss)	2,975				2,975
Purchases of treasury shares, in shares						(1,091,158)
Purchases of treasury shares	(35,569)					$ (35,569)
Balance at Jun. 30, 2016	$ 976,962	$ 3,024	$ 82,836	$ 1,061,391	$ 17,817	$ (188,106)
Balance, in shares, at Jun. 30, 2016		(30,235,164)				(6,241,981)