Consolidated Statements of Stockholders' Equity Statement (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Cumulative Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2010	$ 500,436	$ 36	$ 316,108	$ 281	$ 184,011
Balance,shares at Dec. 31, 2010		36,173
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in unrealized gains and losses on available-for-sale Securities, Net of Tax	9			9
Change in unrealized gains and losses on derivatives, Net of Tax	(267)			(267)
Net Income	91,368				91,368
Share-based Compensation expense	13,727		13,727
Stock Repurchased and Retired During Period, Shares		(25)
Stock Repurchased and Retired During Period, Value	(926)	0			(926)
Issuance of common stock under stock-based compensation plans, shares		1,499
Issuance of common stock under stock-based compensation plans	30,891	2	30,889
Tax benefit from exercises and cancellations of stock options	3,519		3,519
Balance at Dec. 31, 2011	638,757	38	364,243	23	274,453
Balance,shares at Dec. 31, 2011		37,647
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in unrealized gains and losses on available-for-sale Securities, Net of Tax	11			11
Change in unrealized gains and losses on derivatives, Net of Tax	(30)			(30)
Net Income	86,539				86,539
Share-based Compensation expense	14,366		14,366
Stock Repurchased and Retired During Period, Shares		(22)
Stock Repurchased and Retired During Period, Value	(850)	0			(850)
Issuance of common stock under stock-based compensation plans, shares		717
Issuance of common stock under stock-based compensation plans	14,697	0	14,697
Tax benefit from exercises and cancellations of stock options	1,121		1,121
Balance at Dec. 31, 2012	754,611	38	394,427	4	360,142
Balance,shares at Dec. 31, 2012		38,342
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in unrealized gains and losses on available-for-sale Securities, Net of Tax	(24)			(24)
Change in unrealized gains and losses on derivatives, Net of Tax	89			89
Net Income	55,217				55,217
Share-based Compensation expense	17,419		17,419
Stock Repurchased and Retired During Period, Shares	(2,000)	(2,018)
Stock Repurchased and Retired During Period, Value	(63,584)	(1)			(63,583)
Issuance of common stock under stock-based compensation plans, shares		516
Issuance of common stock under stock-based compensation plans	9,626	0	9,626
Tax benefit from exercises and cancellations of stock options	429		429
Balance at Dec. 31, 2013	$ 773,783	$ 37	$ 421,901	$ 69	$ 351,776
Balance,shares at Dec. 31, 2013		36,840