Stock-based Compensation - Company's Options Outstanding (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding - Number of Shares (in shares)	5,196,253
Options Exercisable - Number of Shares (in shares)	3,437,421
$1.92
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 0
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 1.92
Options Outstanding - Number of Shares (in shares)	431,345
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	4 years 2 months 16 days
Options Exercisable - Number of Shares (in shares)	431,345
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	4 years 2 months 16 days
$2.24 – $6.72
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 2.24
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 6.72
Options Outstanding - Number of Shares (in shares)	494,155
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	3 years 10 months 12 days
Options Exercisable - Number of Shares (in shares)	494,155
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	3 years 10 months 12 days
$7.47 – $12.56
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 8.96
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 12.60
Options Outstanding - Number of Shares (in shares)	480,694
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	6 years 9 months 2 days
Options Exercisable - Number of Shares (in shares)	384,717
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	6 years 3 months 12 days
$12.77
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 12.77
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 12.77
Options Outstanding - Number of Shares (in shares)	674,356
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	7 years 22 days
Options Exercisable - Number of Shares (in shares)	488,124
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	7 years 15 days
$12.94
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 12.94
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 12.94
Options Outstanding - Number of Shares (in shares)	577,212
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	8 years 17 days
Options Exercisable - Number of Shares (in shares)	276,718
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	7 years 11 months 27 days
$13.01 – $14.95
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 13.01
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 14.95
Options Outstanding - Number of Shares (in shares)	360,230
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	7 years 7 months 14 days
Options Exercisable - Number of Shares (in shares)	241,553
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	7 years 5 months 2 days
$14.99 – $17.50
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 14.99
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 17.50
Options Outstanding - Number of Shares (in shares)	531,282
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	8 years 3 months 9 days
Options Exercisable - Number of Shares (in shares)	218,634
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	7 years 4 months 10 days
$17.89 – $18.90
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 17.89
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 18.90
Options Outstanding - Number of Shares (in shares)	1,346,556
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	7 years 7 months 16 days
Options Exercisable - Number of Shares (in shares)	806,525
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	6 years 7 months 28 days
$19.09 – $22.71
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Minimum (in dollars per share) | $ / shares	$ 19.09
Exercise Price, Maximum (in dollars per share) | $ / shares	$ 22.71
Options Outstanding - Number of Shares (in shares)	300,423
Options Outstanding -Weighted- Average Remaining Contractual Life in Years	8 years 2 months 20 days
Options Exercisable - Number of Shares (in shares)	95,650
Options Exercisable - Weighted- Average Remaining Contractual Life in Years	7 years 4 months