Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Stock Option Shares
Grants outstanding at beginning of the period	8,806,204	5,436,951		6,515,976
Granted	2,152,439	4,805,722	[1]	1,285,051
Exercised	(2,119,306)	(792,845)		(1,056,821)
Cancelled	(448,166)	(643,624)		(1,307,255)
Grants outstanding at end of the period	8,391,171	8,806,204		5,436,951
Grants exercisable at end of the period	2,304,791	3,288,712		2,942,624
Grants vested and expected to vest at end of the period	6,643,835	6,910,022		5,116,409
Weighted Average Exercise Price
Grants outstanding at beginning of the period	$ 29.74	$ 22.70		$ 23.91
Granted	90.15	35.09		14.89
Exercised	22.89	17.40		18.31
Cancelled	46.11	25.44		24.61
Grants outstanding at end of the period	46.09	29.74		22.70
Grants exercisable at end of the period	27.69	24.76		24.42
Grants vested and expected to vest at end of the period	$ 45.43	$ 28.49		$ 22.83
Aggregate Intrinsic Value
Outstanding at end of year	$ 551,131
Exercisable at end of year	187,693
Vested and expected to vest	$ 433,681
Weighted Average Remaining Contractual Life (Years)
Outstanding at end of year	7 years 8 months 12 days
Exercisable at end of year	5 years 6 months
Vested and expected to vest	7 years 4 months 24 days

[1] In June 2017, the Company granted its new CEO 3,300,000 options with service and market conditions. These options have a five-year cliff vesting schedule. The fair value of $13.48 for these options was determined by a lattice model with Monte Carlo simulations. The remaining are service-based options which have a four-year vesting schedule with 25% vesting on the first anniversary and 1/48 monthly thereafter.