Stock-Based Compensation Expense - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares
Shares (in thousands)
Outstanding, shares | shares	13,789
Granted, shares | shares	2,821
Exercised, shares | shares	(1,239)
Forfeited or expired, shares | shares	(606)
Outstanding, shares | shares	14,765
Exercisable as of reporting date, shares | shares	5,867
Options vested or expected to vest as of reporting date, shares | shares	13,214
Weighted- Average Exercise Price
Outstanding, shares | $ / shares	$ 31.73
Granted, in dollars per share | $ / shares	51.37
Exercised, in dollars per share | $ / shares	21.32
Forfeited or expired, in dollars per share | $ / shares	40.27
Outstanding, shares | $ / shares	36.00
Exercisable as of reporting date, Weighted-Average Exercise Price, in dollars per share | $ / shares	25.12
Weighted- Average Remaining Contractual Term (in years) | $ / shares	$ 34.65
Weighted- Average Remaining Contractual Term (in years)
Outstanding, in years	7 years 5 months 1 day
Exercisable, in years	5 years 11 months 12 days
Options vested or expected to vest, in years	7 years 3 months
Aggregate Intrinsic Value (in thousands)
Outstanding, in dollars | $	$ 207,599
Exercisable, in dollars | $	138,370
Options vested or expected to vest, in dollars | $	$ 201,420