Stockholders' Deficit Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,037	$ 4,317	$ 6,199	$ 12,887
Research and development expenses [Member] | Stock Options, Restricted Stock Units and Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,033	2,246	3,574	6,436
General and administrative expenses [Member] | Stock Options, Restricted Stock Units and Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,004	$ 2,071	$ 2,625	$ 6,451