Summary of Stock-Based Compensation Expense by Grant Type Included within Condensed Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 24,024	$ 16,139
Stock Options [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	15,355	11,457
Restricted Stock Awards/Units (RSAs/RSUs) [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	6,745	3,916
Employee Stock Purchase Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,924	$ 766