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Fair value of financial assets and liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020
Fair value of financial assets and liabilities
Transfer from Leve 1 to Level 2, assets	$ 0	$ 0	$ 0	$ 0
Transfer from Leve 2 to Level 1, assets	0	0	0	0
Transfers into Level 3, assets	0	0	0	0
Transfer out of Level 3, assets	0	0	0	0
Transfer from Leve 1 to Level 2, liabilities	0	0	0	0
Transfer from Leve 2 to Level 1, liabilities	0	0	0	0
Transfers into Level 3, liabilities	0	0	0	0
Transfer out of Level 3, liabilities	0	$ 0	0	$ 0
Recurring
Fair value of financial assets and liabilities
Assets fair value	214,757		214,757		$ 145,767
Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	214,757		214,757		145,767
Money market funds \| Recurring
Fair value of financial assets and liabilities
Assets fair value	13,437		13,437		36,712
Money market funds \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	13,437		13,437		36,712
Commercial paper \| Recurring
Fair value of financial assets and liabilities
Assets fair value					21,884
Commercial paper \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value					21,884
US Treasury bonds \| Recurring
Fair value of financial assets and liabilities
Assets fair value	111,841		111,841		35,810
US Treasury bonds \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	111,841		111,841		35,810
US Government Agency bonds \| Recurring
Fair value of financial assets and liabilities
Assets fair value	89,479		89,479		15,295
US Government Agency bonds \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	$ 89,479		$ 89,479		15,295
Corporate debt securities \| Recurring
Fair value of financial assets and liabilities
Assets fair value					36,066
Corporate debt securities \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value					$ 36,066