Equity Incentive Plan - Stock option (Details) - 2019 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Balance at beginning of period (in shares)	2,136,291	567,319
Granted (in shares)	1,764,713	1,609,086
Exercised (in shares)	(77,368)	(27,832)
Cancelled (in shares)	(81,492)	(12,282)
Balance at end of period (in shares)	3,742,144	2,136,291	567,319
Vested and expected to vest at end of period (in shares)	3,742,144
Exercisable at end of period (in shares)	1,793,254
Vested at end of period (in shares)	1,168,118
Weighted-Average Exercise Price
Balance at beginning of period (in dollars per share)	$ 6.06	$ 3.96
Granted (in dollars per share)	13.63	6.66
Exercised (in dollars per share)	3.94	1.68
Cancelled (in dollars per share)	9.80	4.92
Balance at end of period (in dollars per share)	9.58	$ 6.06	$ 3.96
Vested and expected to vest at end of period (in dollars per share)	9.58
Exercisable at end of period (in dollars per share)	5.87
Vested at end of period (in dollars per share)	$ 6.27
Weighted-Average Remaining Contractual Life
Weighted-Average Remaining Contractual Life	8 years 7 months 6 days	8 years 10 months 24 days	8 years 1 month 6 days
Vested and expected to vest at end of period (in years)	8 years 7 months 6 days
Exercisable at end of period (in years)	7 years 8 months 12 days
Vested at end of period (in years)	7 years 6 months
Aggregate Intrinsic Value
Balance at beginning of period	$ 12,881	$ 694
Balance at end of period	22,910	$ 12,881	$ 694
Vested and expected to vest at end of period	22,910
Exercisable at end of period	17,211
Vested at end of period	$ 10,747