Fair value of financial assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Fair value of financial assets and liabilities
Transfer from Leve 1 to Level 2, assets	$ 0	$ 0
Transfer from Leve 2 to Level 1, assets	0	0
Transfers into Level 3, assets	0	0
Transfer out of Level 3, assets	0	0
Transfer from Leve 1 to Level 2, liabilities	0	0
Transfer from Leve 2 to Level 1, liabilities	0	0
Transfers into Level 3, liabilities	0	0
Transfer out of Level 3, liabilities	0	0
Recurring
Fair value of financial assets and liabilities
Assets fair value	145,767	119,771
Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	145,767	119,771
Money market funds \| Recurring
Fair value of financial assets and liabilities
Assets fair value	36,712	2,676
Money market funds \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	36,712	2,676
Commercial paper \| Recurring
Fair value of financial assets and liabilities
Assets fair value	21,884	46,687
Commercial paper \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	21,884	46,687
US Government Agency bonds \| Recurring
Fair value of financial assets and liabilities
Assets fair value	15,295	20,884
US Government Agency bonds \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	15,295	20,884
US Treasury bonds \| Recurring
Fair value of financial assets and liabilities
Assets fair value	35,810	41,057
US Treasury bonds \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	35,810	41,057
Corporate debt securities \| Recurring
Fair value of financial assets and liabilities
Assets fair value	36,066	8,467
Corporate debt securities \| Level 2 \| Recurring
Fair value of financial assets and liabilities
Assets fair value	$ 36,066	$ 8,467

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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+ References

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-Publisher FASB
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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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+ References

No definition available.

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Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

+ References

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-Publisher FASB
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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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