Earnings Per Share (Details) (USD $) In Thousands, except Per Share data	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Earnings Per Share [Abstract]
Net income	$ 24,169		$ 22,784		$ 82,108		$ 69,443
Weighted average shares outstanding: [Abstract]
Basic (in shares)	47,987		49,615		48,357		49,442
Effect of dilutive securities:
Stock options and employee stock purchase plan shares (in shares)	785		553		800		718
Diluted (in shares)	48,772		50,168		49,157		50,160
Basic Earnings Per Share (in dollars per share)	$ 0.50		$ 0.46		$ 1.70		$ 1.40
Diluted Earnings Per Share (in dollars per share)	$ 0.50		$ 0.45		$ 1.67		$ 1.38
Anti-dilutive stock options whose exercise prices were higher than the common stock's average market price during the period (in shares)	1,418	[1]	2,039	[1]	1,418	[1]	1,460	[1]

[1] Since these options have exercise prices that are higher than the average market prices of our common stock, including them in the calculation would have an anti-dilutive effect on eranings per share.