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Share-based compensation - Dividend equivalent rights - Summary of awards (Details)
Mar. 04, 2021
Mar. 04, 2020
Mar. 07, 2019
Apr. 04, 2018
Jan. 15, 2016
Mar. 06, 2015
Sep. 01, 2014
Mar. 12, 2014
Share-based compensation.                
Valuation Method Black-Scholes Black-Scholes Black-Scholes Black-Scholes Monte Carlo Monte Carlo Monte Carlo Monte Carlo