Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 32,332	$ 21,375	$ 102,305	$ 89,969
Weighted average shares outstanding: [Abstract]
Basic (in shares)	46,380	46,574	46,475	47,076
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	1,218	1,213	1,245	1,129
Diluted (in shares)	47,598	47,787	47,720	48,205
Basic (in dollars per share)	$ 0.70	$ 0.46	$ 2.20	$ 1.91
Diluted (in dollars per share)	$ 0.68	$ 0.45	$ 2.14	$ 1.87
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0	4	0	964