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Equity-settled share-based payments - Key Grant Date Fair Value and Other Assumptions (Details) - USD ($)
May 13, 2021
Apr. 01, 2021
May 12, 2020
Apr. 01, 2020
May 15, 2019
Apr. 01, 2019
Share-Based Payment Arrangements [Abstract]            
Grant date, fair value (USD per share) $ 8.09 $ 8.36 $ 7.45 $ 5.65 $ 7.20 $ 7.00
Possibility of ceasing employment before vesting (as a percent) 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%