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Short-term investments (Details) - Short-term investments - USD ($)
$ in Thousands
12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Short-term investments    
Amortized cost $ 293,739 $ 108,732
Gross unrealized gains 67 388
Gross unrealized losses (22) (65)
Fair value 293,784 109,055
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 67,017 15,217
Gross unrealized gains 12 78
Gross unrealized losses (17)  
Fair value 67,012 15,295
US Treasury bonds    
Short-term investments    
Amortized cost 226,722 35,590
Gross unrealized gains 55 220
Gross unrealized losses (5)  
Fair value $ 226,772 35,810
Corporate debt securities    
Short-term investments    
Amortized cost   36,107
Gross unrealized gains   24
Gross unrealized losses   (65)
Fair value   $ 36,066