Stock-Based Compensation Expense - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Shares (in thousands)
Outstanding, shares | shares	12,726
Granted, shares | shares	2,318
Exercised, shares | shares	(1,434)
Forfeited or expired, shares | shares	(429)
Outstanding, shares | shares	13,181
Exercisable as of reporting date, shares | shares	4,327
Options vested or expected to vest as of reporting date, shares | shares	11,916
Weighted- Average Exercise Price
Outstanding, shares | $ / shares	$ 25.24
Granted, in dollars per share | $ / shares	55.66
Exercised, in dollars per share | $ / shares	18.06
Forfeited or expired, in dollars per share | $ / shares	33.24
Outstanding, shares | $ / shares	31.11
Exercisable as of reporting date, Weighted-Average Exercise Price, in dollars per share | $ / shares	20.14
Weighted- Average Remaining Contractual Term (in years) | $ / shares	$ 29.99
Weighted- Average Remaining Contractual Term (in years)
Outstanding, in years	7 years 5 months 23 days
Exercisable, in years	6 years 4 days
Options vested or expected to vest, in years	7 years 4 months 13 days
Aggregate Intrinsic Value (in thousands)
Outstanding, in dollars | $	$ 329,301
Exercisable, in dollars | $	154,377
Options vested or expected to vest, in dollars | $	$ 310,772