Stock-Based Compensation Expense - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Shares (in thousands)
Outstanding, shares | shares	13,789
Granted, shares | shares	2,761
Exercised, shares | shares	(717)
Forfeited or expired, shares | shares	(139)
Outstanding, shares | shares	15,694
Exercisable as of reporting date, shares | shares	6,289
Options vested or expected to vest as of reporting date, shares | shares	13,860
Weighted- Average Exercise Price
Outstanding, in dollars per share | $ / shares	$ 31.73
Granted, in dollars per share | $ / shares	51.49
Exercised, in dollars per share | $ / shares	20.03
Forfeited or expired, in dollars per share | $ / shares	41.73
Outstanding, in dollars per share | $ / shares	35.65
Exercisable as of reporting date, Weighted-Average Exercise Price, in dollars per share | $ / shares	24.86
Options vested or expected to vest as of period end (in dollars per share) | $ / shares	$ 34.21
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, in years	7 years 7 months 17 days
Exercisable, in years	6 years 2 months 1 day
Options vested or expected to vest, in years	7 years 5 months 16 days
Aggregate Intrinsic Value (in thousands)
Outstanding, in dollars | $	$ 255,322
Exercisable, in dollars | $	166,524
Options vested or expected to vest, in dollars | $	$ 244,429