Stock-Based Compensation - Summary of Options Outstanding and Vested Under Stock Plans by Exercise Price (Detail)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
$1.44 — $18.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Exercise Price Lower Range Limit	$ 1.44
Options Outstanding, Exercise Price Upper Range Limit	$ 18.00
Number of Options Outstanding | shares	717,786
Options Outstanding, Weighted Average Remaining Contractual Life (in Years)	4 years 3 months 14 days
Options Outstanding, Weighted Average Exercise Price	$ 8.62
Options Vested, Number Exercisable | shares	717,786
Options Vested, Weighted Average Exercise Price	$ 8.62
$32.51 — $53.70 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Exercise Price Lower Range Limit	32.51
Options Outstanding, Exercise Price Upper Range Limit	$ 53.70
Number of Options Outstanding | shares	517,192
Options Outstanding, Weighted Average Remaining Contractual Life (in Years)	7 years 3 months 25 days
Options Outstanding, Weighted Average Exercise Price	$ 43.87
Options Vested, Number Exercisable | shares	315,285
Options Vested, Weighted Average Exercise Price	$ 44.40
$54.50 — $63.23 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Exercise Price Lower Range Limit	54.50
Options Outstanding, Exercise Price Upper Range Limit	$ 63.23
Number of Options Outstanding | shares	543,534
Options Outstanding, Weighted Average Remaining Contractual Life (in Years)	6 years 11 months 15 days
Options Outstanding, Weighted Average Exercise Price	$ 58.63
Options Vested, Number Exercisable | shares	362,733
Options Vested, Weighted Average Exercise Price	$ 59.17
$63.39 — $76.81 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Exercise Price Lower Range Limit	63.39
Options Outstanding, Exercise Price Upper Range Limit	$ 76.81
Number of Options Outstanding | shares	591,830
Options Outstanding, Weighted Average Remaining Contractual Life (in Years)	8 years 7 months 2 days
Options Outstanding, Weighted Average Exercise Price	$ 73.13
Options Vested, Number Exercisable | shares	221,697
Options Vested, Weighted Average Exercise Price	$ 72.83
$86.90 — $97.52 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Exercise Price Lower Range Limit	86.90
Options Outstanding, Exercise Price Upper Range Limit	$ 97.52
Number of Options Outstanding | shares	159,398
Options Outstanding, Weighted Average Remaining Contractual Life (in Years)	7 years 10 months 20 days
Options Outstanding, Weighted Average Exercise Price	$ 88.84
Options Vested, Number Exercisable | shares	82,882
Options Vested, Weighted Average Exercise Price	$ 89.05
$1.44 — $97.52 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Exercise Price Lower Range Limit	1.44
Options Outstanding, Exercise Price Upper Range Limit	$ 97.52
Number of Options Outstanding | shares	2,529,740
Options Outstanding, Weighted Average Remaining Contractual Life (in Years)	6 years 8 months 19 days
Options Outstanding, Weighted Average Exercise Price	$ 46.72
Options Vested, Number Exercisable | shares	1,700,383
Options Vested, Weighted Average Exercise Price	$ 38.33