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Stock-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Closing stock price (in USD per share)	$ 18.37		$ 18.37
Service-based Vesting Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation cost	$ 0.5	$ 0.6	$ 1.1	$ 1.2
Unrecognized stock-based compensation cost	$ 2.1		$ 2.1
Unrecognized stock-based compensation cost, expected weighted-average period of recognition			2 years 3 months 18 days
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested RSAs outstanding (in shares)	1,658,396		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	$ 4.9	$ 3.4	$ 8.9	$ 6.7
Unrecognized stock-based compensation cost, expected weighted-average period of recognition			2 years 4 months 24 days
Unvested RSAs outstanding (in shares)	2,994,375		2,994,375		2,311,768
Unrecognized stock-based compensation cost	$ 32.8		$ 32.8