Condensed Consolidated Statements of Stockholders' Equity (USD $) In Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings
Beginning Balance at Dec. 31, 2011	$ 996,509	$ 430	$ 726,956	$ (12,350)	$ 281,473
Stock issued for benefit plans	1,654		354	1,300
Stock options exercised	636	1	635
Purchase of treasury shares	(2,805)			(2,805)
Tax benefits from stock compensation	175		175
Employee stock purchase plan	1,076		1,076
Issuance of restricted stock		4	(4)
Amortization of share-based compensation	18,676		18,676
Net Income	20,939				20,939
Ending Balance at Dec. 31, 2012	1,036,860	435	747,868	(13,855)	302,412
Stock issued for benefit plans	1,622		(1,171)	2,793
Purchase of treasury shares	(1,433)			(1,433)
Tax benefits from stock compensation	(1,568)		(1,568)
Employee stock purchase plan	946	1	945
Issuance of restricted stock		3	(3)
Amortization of share-based compensation	9,232		9,232
Net Income	13,931				13,931
Ending Balance at Jun. 30, 2013	$ 1,059,590	$ 439	$ 755,303	$ (12,495)	$ 316,343