Short-term investments (Details) - Short-term investments - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Sep. 30, 2020	Mar. 31, 2020
Short-term investments
Amortized cost	$ 201,240	$ 108,732
Gross unrealized gains	82	388
Gross unrealized losses	(2)	(65)
Fair value	201,320	109,055
Commercial paper
Short-term investments
Amortized cost		21,818
Gross unrealized gains		66
Fair value		21,884
US Government Agency bonds
Short-term investments
Amortized cost	89,462	15,217
Gross unrealized gains	19	78
Gross unrealized losses	(2)
Fair value	89,479	15,295
US Treasury bonds
Short-term investments
Amortized cost	111,778	35,590
Gross unrealized gains	63	220
Fair value	$ 111,841	35,810
Corporate debt securities
Short-term investments
Amortized cost		36,107
Gross unrealized gains		24
Gross unrealized losses		(65)
Fair value		$ 36,066

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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Amount of unrealized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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