Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - 2016 Equity Incentive Plans and Employee Stock Purchase Plan - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,081	$ 835
Research and Development
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	402	323
General and Administrative
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 679	$ 512