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Equity (Share-based Payments to Employees, Non-marketable Options, Movement) (Details) - Non-marketable options [Member]
Pure in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of terms and conditions of share-based payment arrangement [Line Items]    
Balance as at start of year 18 20
Granted during the year 19 6
Expired during the year (3) (6)
Forfeited during the year (3) (1)
Exercised during the year (1) (1)
Balance as at end of year 30 18