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Derivative financial instruments (Details)
12 Months Ended
Jul. 23, 2020
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2018
Disclosure of detailed information about financial instruments [line items]        
Hedging percentage 90.00%   90.00% 90.00%
Hedging instrument, fiscal year 2021 40 30    
Commodity risk | Maximum        
Disclosure of detailed information about financial instruments [line items]        
Hedging percentage   90.00%