Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 73,863	$ 66,533	$ 78,051	$ 69,973
Weighted average shares outstanding: [Abstract]
Basic (in shares)	44,769	46,659	44,972	46,523
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	1,202	1,223	1,188	1,235
Diluted (in shares)	45,971	47,882	46,160	47,758
Basic (in dollars per share)	$ 1.65	$ 1.43	$ 1.74	$ 1.50
Diluted (in dollars per share)	$ 1.61	$ 1.39	$ 1.69	$ 1.47
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0	0	169	1