Common Stock and Stock-Based Awards - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Equity [Abstract]
Number of Shares, Beginning Balance | shares	8,310,683
Number of Shares, Granted | shares	4,574,365
Number of Shares, Exercised | shares	(1,969,308)
Number of Shares, Forfeited | shares	(1,113,107)
Number of Shares, Ending Balance | shares	9,802,633	8,310,683
Number of Shares, Options exercisable | shares	3,264,289
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 10.36
Weighted Average Exercise Price, Granted | $ / shares	4.59
Weighted Average Exercise Price, Exercised | $ / shares	6.15
Weighted Average Exercise Price, Forfeited | $ / shares	10.66
Weighted Average Exercise Price, Ending Balance | $ / shares	8.47	$ 10.36
Weighted Average Exercise Price, Options exercisable | $ / shares	$ 14.60
Weighted Average Remaining Contractual Term, Outstanding	7 years 11 months 4 days	7 years 3 days
Weighted Average Remaining Contractual Term, Options exercisable	5 years 10 months 28 days
Aggregate Intrinsic Value, Outstanding | $	$ 196,033	$ 3,427
Aggregate Intrinsic Value, Options exercisable | $	$ 46,335