Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 66,533	$ 64,943	$ 69,973	$ 68,594
Weighted average shares outstanding: [Abstract]
Basic (in shares)	46,659	47,142	46,523	47,330
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	1,223	1,146	1,235	1,100
Diluted (in shares)	47,882	48,288	47,758	48,430
Basic (in dollars per share)	$ 1.43	$ 1.38	$ 1.50	$ 1.45
Diluted (in dollars per share)	$ 1.39	$ 1.34	$ 1.47	$ 1.42
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0	991	1	1,174