Stock-Based Compensation - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		12 Months Ended
	May 03, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Stock Based Compensation
Antidilutive securities excluded from computation of diluted net loss per share			17,612,763
2015 Plan
Number of Shares
Outstanding at the beginning (in shares)		2,170,212
Granted (in shares)		884,525
Exercised (in shares)		(17,142)
Forfeitures (in shares)		(22,000)
Outstanding at the end (in shares)		3,015,595		2,170,212
Vested and expected to vest at the end (in shares)		3,015,595
Exercisable at the end (in shares)		828,250
Weighted Average Exercise Price
Outstanding at the beginning (in dollars per share)		$ 6.51
Granted (in dollars per share)		41.65
Exercised (in dollars per share)		2.08
Forfeitures (in dollars per share)		15.57
Outstanding at the end (in dollars per share)		16.77		$ 6.51
Vested and expected to vest at the end (in dollars per share)		16.77
Exercisable at the end (in dollars per share)		$ 3.97
Weighted Average Remaining Contractual Life (Years) And Aggregate Intrinsic Value
Outstanding (in years)		9 years		8 years 7 months 6 days
Granted (in years)		9 years 10 months 24 days
Exercised (in years)		7 years 6 months
Forfeitures (in years)		9 years 2 months 12 days
Outstanding at the beginning (in dollars)		$ 113,295
Outstanding at the end (in dollars)		65,322		$ 113,295
Vested and expected to vest at the end (in dollars)		65,322
Exercisable at the end (in dollars)		$ 27,417
Weighted average grant date fair value per share of stock options granted		$ 25.90
Aggregate intrinsic value of stock options exercised		$ 600
Aggregate grant date fair value		30,300	$ 7,600
Share Based Compensation Expense Not Recognized
Unrecognized compensation cost		$ 28,600
Compensation expense recognition period		3 years
Omnibus Incentive Plan
Stock Based Compensation
Number of shares reserved for issuance under the plan	2,900,000
Annual increase in number of share reserved for issuance (as percent)	4.00%
2015 Plan and 2019 Employee Stock Purchase Plan
Stock Based Compensation
Vesting period		4 years
Expiration period		10 years
2019 Employee Stock Purchase Plan
Stock Based Compensation
Number of shares reserved for issuance under the plan		2,899,011
Option to purchase common stock
Stock Based Compensation
Antidilutive securities excluded from computation of diluted net loss per share			2,052,194