Stock‑Based Compensation - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average grant-date fair value of stock options granted	$ 17.81	$ 1.22	$ 7.08	$ 1.10
Stock option
Number of Shares
Outstanding as at beginning of period (in shares)			961,751
Granted (in shares)			1,989,654
Exercised (in shares)			(32,511)
Forfeited (in shares)			(57,766)
Outstanding as at end of period (in shares)	2,861,128		2,861,128		961,751
Vested and expected to vest as at end of period (in shares)	2,861,128		2,861,128
Options exercisable as at end of period (in shares)	349,736		349,736
Weighted Average Exercise Price
Weighted average exercise price outstanding as at beginning of period (in dollars per share)			$ 1.69
Weighted average exercise price, Granted (in dollars per share)			8.67
Weighted average exercise price, Exercised (in dollars per share)			1.27
Weighted average exercise price, Forfeited (in dollars per share)			3.32
Weighted average exercise price, Outstanding as at end of period (in dollars per share)	$ 6.54		6.54		$ 1.69
Vested and expected to vest as at end of period (in dollars per share)	6.54		6.54
Options exercisable as at end of period (in dollars per share)	$ 3.58		$ 3.58
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Contractual term, Outstanding (in years)			9 years 2 months 12 days		9 years 6 months
Contractual term, Vested and expected to vest (in years)			9 years 2 months 12 days
Contractual term, Options exercisable (in years)			8 years 7 months 6 days
Average intrinsic value, Outstanding	$ 46,726		$ 46,726		$ 524
Average intrinsic value, Vested and expected to vest	46,726		46,726
Average intrinsic value, Exercisable	$ 6,745		$ 6,745