Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - 2008 Plan, 2015 Plan and 2018 Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	51,514
Shares Available for Grant, Awards authorized	4,887,187
Shares Available for Grant, Granted	(3,965,439)
Shares Available for Grant, forfeited/cancelled	45,295
Shares Available for Grant, Ending balance	1,018,557	51,514
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	326,023
Number of Shares Outstanding, Options granted	3,965,439
Number of Shares Outstanding, Options forfeited/cancelled	(45,295)
Number of Shares Outstanding, Ending balance	4,246,167	326,023
Number of Shares Outstanding, Exercisable	413,241
Number of Shares Outstanding, Vested and expected to vest	3,216,792
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 102.56
Weighted-Average Exercise Price Per Share, Options granted	12.84
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled	45.35
Weighted-Average Exercise Price Per Share, Ending balance	19.38	$ 102.56
Weighted-Average Exercise Price Per Share, Exercisable	70.97
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 21.53
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 8 months 15 days	7 years 5 months 12 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	3 years 11 months 12 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 9 months 29 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 3	$ 11
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 3