Stockholders' Equity - Summary of Stock-Based Compensation Expense (Details) - 2016 Equity Incentive Plans and Employee Stock Purchase Plan - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,366	$ 1,072	$ 3,692	$ 2,903
Research and Development
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	492	377	1,358	1,090
General and Administrative
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 874	$ 695	$ 2,334	$ 1,813