Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - 2008 Plan, 2015 Plan and 2018 Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 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	(2,055,130)
Shares Available for Grant, forfeited/cancelled	30,098
Shares Available for Grant, Ending balance	2,913,669	51,514
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	326,023
Number of Shares Outstanding, Options granted	2,055,130
Number of Shares Outstanding, Options forfeited/cancelled	(30,098)
Number of Shares Outstanding, Ending balance	2,351,055	326,023
Number of Shares Outstanding, Exercisable	358,948
Number of Shares Outstanding, Vested and expected to vest	2,152,111
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	13.51
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled	57.08
Weighted-Average Exercise Price Per Share, Ending balance	25.30	$ 102.56
Weighted-Average Exercise Price Per Share, Exercisable	78.02
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 26.27
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	7 years 11 months 23 days	7 years 5 months 12 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	3 years 11 months 4 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 6 months 21 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 9	$ 11
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 9