Net Income per Share - Schedule of Components Used in Diluted and Basic Income Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income	$ 97,107	$ 91,155	$ 72,886	$ 72,800	$ 80,349	$ 79,756	$ 61,895	$ 71,487	$ 333,948	$ 293,487	$ 203,989
Denominator:
Shares used for basic net income per share (in shares)									178,279	178,196	177,900
Effect of dilutive securities:
Stock options (in shares)									1,221	1,622	2,182
RSUs and deferred stock units (in shares)									1,686	1,965	1,667
Convertible senior notes (in shares)									0	0	0
Warrants related to issuance of convertible senior notes (in shares)									0	0	0
Shares used for diluted net income per share (in shares)									181,186	181,783	181,749
Basic net income per share (in dollars per share)	$ 0.55	$ 0.51	$ 0.41	$ 0.41	$ 0.45	$ 0.45	$ 0.35	$ 0.40	$ 1.87	$ 1.65	$ 1.15
Diluted net income per share (in dollars per share)	$ 0.54	$ 0.50	$ 0.40	$ 0.40	$ 0.44	$ 0.44	$ 0.34	$ 0.39	$ 1.84	$ 1.61	$ 1.12