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Dividends (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Feb. 20, 2018
$ / shares
Feb. 20, 2018
R / shares
Feb. 21, 2017
$ / shares
Feb. 21, 2017
R / shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Disclosure of share capital, reserves and other equity interest [Abstract]            
Dividends declared per share | (per share) $ 0.06 R 0.70 $ 0.1 R 1.30    
Dividends paid         $ 24 $ 39