Stock-based compensation - PSU (Details) $ in Thousands	1 Months Ended				3 Months Ended		8 Months Ended	24 Months Ended
	Sep. 30, 2020 milestone installment shares	Feb. 29, 2020 installment milestone shares	Sep. 30, 2019 milestone shares	Oct. 31, 2018 milestone shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 shares	Sep. 30, 2020 shares	Nov. 30, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)					575,000
Total stock-based compensation expense | $					$ 2,857	$ 1,752
Equity Incentive Plan 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares reserved for future issuance (up to) (in shares)	5,800,000				5,800,000		5,800,000	5,800,000	5,800,000
PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance milestones | milestone	4	4	4	4
Trading days, period	30 days	30 days	30 days	30 days
Performance period (in years)			3 years	3 years
Performance-based RSUs earned (in shares)	0				0		0	0
Percentage eligible to vest	25.00%	25.00%	25.00%	25.00%
Percentage that will vest upon reaching milestone			50.00%	50.00%
Percentage will vest on one year anniversary of performance milestone			50.00%	50.00%
Total stock-based compensation expense | $					$ 1,100
Performance-based RSUs canceled (in shares)					0
Performance-based RSUs outstanding (in shares)	1,495,000				1,495,000		1,495,000	1,495,000
PSU | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	195,000	500,000	560,000	240,000
Performance period (in years)	5 years	4 years
Number of installments | installment	4	4
PSU | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period (in years)	4 years	3 years