Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) item $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares
Numerator
Net income | $	$ 99,188	$ 93,276	$ 285,050	$ 277,937
Denominator
Weighted-average common shares outstanding	103,221,692	102,842,996	103,117,695	102,786,822
Basic net income per share (in dollars per share) | $ / shares	$ 0.96	$ 0.91	$ 2.76	$ 2.70
Numerator
Net income | $	$ 99,188	$ 93,276	$ 285,050	$ 277,937
Assumed conversion of convertible senior notes | $	1,426	1,472	4,263	4,382
Net income plus assumed conversions | $	$ 100,614	$ 94,748	$ 289,313	$ 282,319
Denominator
Number of shares used in basic computation	103,221,692	102,842,996	103,117,695	102,786,822
Weighted-average effect of dilutive securities (in shares)	8,487,853	7,945,917	8,440,430	7,951,067
Number of shares used in per share computation	111,709,545	110,788,913	111,558,125	110,737,889
Diluted net income per share (in dollars per share) | $ / shares	$ 0.90	$ 0.86	$ 2.59	$ 2.55
Number of classes of common stock | item			2
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares)			1,085,559	994,001