Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share [Abstract]
Net income	$ 64,943	$ 58,577	$ 68,594	$ 57,939
Weighted average shares outstanding: [Abstract]
Basic (in shares)	47,142	48,231	47,330	48,546
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	1,146	885	1,100	806
Diluted (in shares)	48,288	49,116	48,430	49,352
Basic (in dollars per share)	$ 1.38	$ 1.21	$ 1.45	$ 1.19
Diluted (in dollars per share)	$ 1.34	$ 1.19	$ 1.42	$ 1.17
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	991	1,384	1,174	1,420