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Stock-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Closing stock price (in USD per share)	$ 19.07
Service-based Vesting Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation cost	$ 0.6
Unrecognized stock-based compensation cost	$ 2.7
Unrecognized stock-based compensation cost, expected weighted-average period of recognition	1 year 10 months 24 days
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested RSAs outstanding (in shares)	1,658,396
Restricted Stock Awards and Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation cost	$ 4.1	$ 3.3
Unrecognized stock-based compensation cost, expected weighted-average period of recognition	2 years 6 months
Unvested RSAs outstanding (in shares)	3,202,323		2,311,768
Unrecognized stock-based compensation cost	$ 36.8