Stock-Based Compensation Expense - Categorized by Equity Components (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 33,462	$ 21,101	$ 11,517
Employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1,980	3,249	2,676
Restricted stock awards and units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	28,909	17,024	8,232
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,573	$ 828	$ 609