Statement of Stockholders' Equity (USD $) In Thousands, except Share data	Total	Common Stock Member	Additional Paid In Capital Member	Retained Earnings Member	Treasury Stock Member	Accumulated Other Comprehensive Income Member
Beginning balance - Value at Dec. 31, 2009	$ 481,360	$ 359	$ 326,224	$ 148,620	$ (8)	$ 6,165
Beginning balance - Shares at Dec. 31, 2009		35,919,941			(417)
Comprehensive income:
Net income (unaudited)	15,607			15,607
Change in unrealized gain on available-for-sale securities, net of taxes of $100 (unaudited)	602					602
Total comprehensive income (unaudited)	16,209
Tax expense related to exercise of stock options (unaudited)	286		286
Stock-based compensation expense recognized in earnings (unaudited)	(3,166)		3,166
Issuance of stock related to stock-based awards (unaudited) - Value	598	2	596
Issuance of stock related to stock-based awards (unaudited) - Shares		125,077
Issuance of common stock (unaudited) - Value	12,459	7	12,452
Issuance of common stock (unaudited) - Shares		732,235
Ending balance (unaudited) - Value at Jun. 30, 2010	514,078	368	342,724	164,227	(8)	6,767
Ending balance (unaudited) - Shares at Jun. 30, 2010		36,777,253			(417)
Beginning balance - Value at Dec. 31, 2010	528,319	369	336,796	185,807	(8)	5,355
Beginning balance - Shares at Dec. 31, 2010		36,957,104			(417)
Comprehensive income:
Net income (unaudited)	28,533			28,533
Change in unrealized gain on available-for-sale securities, net of taxes of $100 (unaudited)	(133)					(133)
Total comprehensive income (unaudited)	28,400
Tax expense related to exercise of stock options (unaudited)	1,616		1,616
Stock-based compensation expense recognized in earnings (unaudited)	(4,185)		4,185
Issuance of stock related to stock-based awards (unaudited) - Value	1,404	4	1,400
Issuance of common stock (unaudited) - Shares		373,039
Ending balance (unaudited) - Value at Jun. 30, 2011	$ 563,924	$ 373	$ 343,997	$ 214,340	$ (8)	$ 5,222
Ending balance (unaudited) - Shares at Jun. 30, 2011		37,330,143			(417)