Net Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 item	Mar. 31, 2012
Net Earnings Per Share
Number of classes of common stock	2
Numerator
Net income	$ 39,996	$ 26,927
Denominator
Weighted-average common shares outstanding	101,260,614	100,717,302
Basic net income per share (in dollars per share)	$ 0.39	$ 0.27
Numerator
Net income	39,996	26,927
Assumed conversion of convertible senior notes	1,407	1,480
Net income plus assumed conversions	$ 41,403	$ 28,407
Denominator
Number of shares used in basic computation	101,260,614	100,717,302
Weighted-average effect of dilutive securities (in shares)	7,086,271	6,709,487
Number of shares used in per share computation	108,346,885	107,426,789
Diluted net income per share (in dollars per share)	$ 0.38	$ 0.26
Stock options
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)	150,000	3,365,908
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)	2,192,931	2,970,643