Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Stock Option Shares
Grants outstanding at beginning of the period	5,436,951		6,515,976	5,216,203
Granted	4,805,722	[1]	1,285,051	2,830,078
Exercised	(792,845)		(1,056,821)	(816,696)
Cancelled	(643,624)		(1,307,255)	(713,609)
Grants outstanding at end of the period	8,806,204		5,436,951	6,515,976
Grants exercisable at end of the period	3,288,712		2,942,624	2,617,167
Grants vested and expected to vest at end of the period	6,910,022		5,116,409	5,908,213
Weighted Average Exercise Price
Grants outstanding at beginning of the period	$ 22.70		$ 23.91	$ 24.45
Granted	35.09		14.89	20.28
Exercised	17.40		18.31	12.26
Cancelled	25.44		24.61	26.78
Grants outstanding at end of the period	29.74		22.70	23.91
Grants exercisable at end of the period	24.76		24.42	23.85
Grants vested and expected to vest at end of the period	$ 28.49		$ 22.83	$ 23.94
Aggregate Intrinsic Value
Outstanding at end of year	$ 228,081
Exercisable at end of year	101,559
Vested and expected to vest	$ 187,582
Weighted Average Remaining Contractual Life (Years)
Outstanding at end of year	7 years 7 months 6 days
Exercisable at end of year	5 years 1 month 6 days
Vested and expected to vest	7 years 1 month 6 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.