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Equity-settled share-based payments - Key Grant Date Fair Value and Other Assumptions (Details) - USD ($)
May 15, 2019
Apr. 01, 2019
May 15, 2018
Apr. 01, 2018
May 15, 2017
Apr. 01, 2017
Share-Based Payment Arrangements [Abstract]            
Grant date, fair value (USD per share) $ 7.20 $ 7.00 $ 9.10 $ 8.43 $ 12.14 $ 11.88
Possibility of ceasing employment before vesting (as a percent) 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%