Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Accumulated Other Non-Owner Changes to Equity [Member]
Balance at Dec. 31, 2014	$ 1,111,793	$ 612	$ 405,525	$ (169,405)	$ 974,514	$ (99,453)
Balance (in shares) at Dec. 31, 2014		61,230,000		6,729,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	77,581				121,380	(43,799)
Dividends paid	(26,176)				(26,176)
Common stock repurchases	(52,103)			$ (52,103)
Common stock repurchases (in shares)				1,352,596
Employee stock plans		$ 9
Employee stock plans (in shares)		841,164
Employee stock plans, other			22,033
Employee stock plans, shares withheld for taxes (in shares)				125,000
Employee stock plans, shares withheld for taxes				$ (4,913)
Dividends paid to holders of certain restricted stock units					(471)
Employee stock plans, total	16,658
Balance at Dec. 31, 2015	1,127,753	$ 621	427,558	$ (226,421)	1,069,247	(143,252)
Balance (in shares) at Dec. 31, 2015		62,071,000		8,207,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	78,025				135,601	(57,576)
Dividends paid	(27,435)				(27,435)
Common stock repurchases	(20,520)			$ (20,520)
Cumulative effect of change in accounting guidance (Note 12)	198				198
Common stock repurchases (in shares)				550,994
Employee stock plans		$ 6
Employee stock plans (in shares)		621,259
Employee stock plans, other			15,677
Employee stock plans, shares withheld for taxes (in shares)				132,000
Employee stock plans, shares withheld for taxes				$ (4,886)
Dividends paid to holders of certain restricted stock units					(460)
Employee stock plans, total	10,337
Balance at Dec. 31, 2016	1,168,358	$ 627	443,235	$ (251,827)	1,177,151	(200,828)
Balance (in shares) at Dec. 31, 2016		62,692,000		8,889,994
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	153,844				59,415	94,429
Dividends paid	(29,551)				(29,551)
Common stock repurchases	(40,791)			$ (40,791)
Common stock repurchases (in shares)				677,100
Employee stock plans		$ 3
Employee stock plans (in shares)		341,837
Employee stock plans, other			14,130
Employee stock plans, shares withheld for taxes (in shares)				89,000
Employee stock plans, shares withheld for taxes				$ (5,380)
Dividends paid to holders of certain restricted stock units					(292)
Employee stock plans, total	8,461
Balance at Dec. 31, 2017	$ 1,260,321	$ 630	$ 457,365	$ (297,998)	$ 1,206,723	$ (106,399)
Balance (in shares) at Dec. 31, 2017		63,034,000		9,656,094