Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) item $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares
Anti-dilutive securities
Number of classes of common stock | item			2
Numerator
Net income | $	$ 115,211	$ 100,925	$ 225,862	$ 185,862
Denominator
Weighted-average common shares outstanding	104,003,960	103,180,769	103,876,647	103,064,834
Basic net income per share (in dollars per share) | $ / shares	$ 1.11	$ 0.98	$ 2.17	$ 1.80
Numerator
Net income | $	$ 115,211	$ 100,925	$ 225,862	$ 185,862
Assumed conversion of convertible senior notes | $	1,735	1,431	3,474	2,847
Net income plus assumed conversions | $	$ 116,946	$ 102,356	$ 229,336	$ 188,709
Denominator
Number of shares used in basic computation	104,003,960	103,180,769	103,876,647	103,064,834
Weighted-average effect of dilutive securities (in shares)	8,420,622	8,383,714	8,449,859	8,425,849
Number of shares used in per share computation	112,424,582	111,564,483	112,326,506	111,490,683
Diluted net income per share (in dollars per share) | $ / shares	$ 1.04	$ 0.92	$ 2.04	$ 1.69
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)			951,878	1,086,653