Equity Incentive Plan - Stock option (Details) - 2019 Equity Incentive Plan	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Number of Shares
Balance at beginning of period (in shares) | shares	2,136,291
Granted (in shares) | shares	1,658,012
Exercised (in shares) | shares	(67,243)
Cancelled (in shares) | shares	(36,662)
Balance at end of period (in shares) | shares	3,690,398	2,136,291
Vested and expected to vest at end of period (in shares) | shares	3,690,398
Exercisable at end of period (in shares) | shares	1,736,235
Vested at end of period (in shares) | shares	993,078
Weighted-Average Exercise Price
Balance at beginning of period (in dollars per share) | $ / shares	$ 6.06
Granted (in dollars per share) | $ / shares	13.68
Exercised (in dollars per share) | $ / shares	3.83
Cancelled (in dollars per share) | $ / shares	11.03
Balance at end of period (in dollars per share) | $ / shares	9.46	$ 6.06
Vested and expected to vest at end of period (in dollars per share) | $ / shares	9.46
Exercisable at end of period (in dollars per share) | $ / shares	5.56
Vested at end of period (in dollars per share) | $ / shares	$ 5.91
Weighted-Average Remaining Contractual Life
Weighted-Average Remaining Contractual Life	8 years 9 months 18 days	8 years 10 months 24 days
Vested and expected to vest at end of period (in years)	8 years 9 months 18 days
Exercisable at end of period (in years)	8 years 1 month 6 days
Vested at end of period (in years)	7 years 9 months 18 days
Aggregate Intrinsic Value
Balance at end of period | $	$ 12,830	$ 12,881
Vested and expected to vest at end of period | $	12,830
Exercisable at end of period | $	11,606
Vested at end of period | $	$ 6,296