Condensed Consolidated Statements of Stockholders' Equity (USD $) In Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2009	$ 678,899	$ 379	$ 551,606	$ (9,221)	$ 136,358	$ (223)
Stock issued for benefit plans	1,513		242	1,271
Stock options exercised	2,087	1	2,086
Public stock offering	140,139	40	140,099	0	0	0
Purchase of treasury shares	(1,828)			(1,828)
Tax benefits from stock-based compensation	28		28
Employee stock purchase plan	951	1	950
Issuance of restricted stock		3	(3)
Amortization of share-based compensation	11,849		11,849
Net Income	46,294				46,294
Other comprehensive income	85					85
Ending Balance at Dec. 31, 2010	880,017	424	706,857	(9,778)	182,652	(138)
Stock issued for benefit plans	1,612		791	821
Stock options exercised	1,151	1	1,150
Purchase of treasury shares	(3,393)			(3,393)
Tax benefits from stock-based compensation	333		333
Employee stock purchase plan	1,000	1	999
Issuance of restricted stock		4	(4)
Amortization of share-based compensation	16,830		16,830
Net Income	98,821				98,821
Other comprehensive income	138					138
Ending Balance at Dec. 31, 2011	996,509	430	726,956	(12,350)	281,473
Stock issued for benefit plans	1,654	0	354	1,300	0	0
Stock options exercised	636	1	635	0	0	0
Purchase of treasury shares	(2,805)	0	0	(2,805)	0	0
Tax benefits from stock-based compensation	175	0	175	0	0	0
Employee stock purchase plan	1,076	0	1,076	0	0	0
Issuance of restricted stock		4	(4)	0	0	0
Amortization of share-based compensation	18,676	0	18,676	0	0	0
Net Income	20,939	0	0	0	20,939	0
Other comprehensive income	0	0	0	0	0	0
Ending Balance at Dec. 31, 2012	$ 1,036,860	$ 435	$ 747,868	$ (13,855)	$ 302,412