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, 2010
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, 2011	996,509	430	726,956	(12,350)	281,473
Beginning 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	(1,409)			(1,409)
Tax benefits from stock compensation	(1,587)		(1,587)
Employee stock purchase plan	946	1	945
Issuance of restricted stock		3	(3)
Amortization of share-based compensation	4,645		4,645
Net Income	7,209				7,209
Ending Balance at Mar. 31, 2013	$ 1,048,286	$ 439	$ 750,697	$ (12,471)	$ 309,621