Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options
Options, Beginning Balance	2,992,724
Options, Granted	618,899	166,272	2,542,523
Options, Forfeited	(177,064)
Options, Exercised	(728,710)
Options, Ending Balance	2,705,849	2,992,724
Options, Vested and expected to vest	2,639,619
Options, Exercisable	1,286,981
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 22.43
Weighted-Average Exercise Price, Granted	38.49
Weighted-Average Exercise Price, Forfeited	30.91
Weighted-Average Exercise Price, Exercised	22.47
Weighted-Average Exercise Price, Ending Balance	25.53	$ 22.43
Weighted-Average Exercise Price, Vested and expected to vest	25.53
Weighted-Average Exercise Price, Exercisable	$ 19.14
Aggregate Intrinsic Value/Weighted-Average Exercise Term
Aggregate Intrinsic Value	$ 125,197	$ 46,608
Aggregate Intrinsic Value, Vested and expected to vest	121,850
Aggregate Intrinsic Value, Exercisable	$ 67,770
Weighted-Average Exercise Term, Outstanding Balance	7 years 3 months 10 days	7 years 8 months 4 days
Weighted-Average Exercise Term, Vested and expected to vest	7 years 3 months 10 days
Weighted-Average Exercise Term, Exercisable	6 years 4 months 9 days