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Mine restoration provisions - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Disclosure of fair value measurement of assets [line items] |  |
Inflation rate | 2.10% |
Current estimate to settle mine restoration provisions | $ 126 | $ 102
Bottom of range |  |
Disclosure of fair value measurement of assets [line items] |  |
Discount rate | 2.50% |
Top of range |  |
Disclosure of fair value measurement of assets [line items] |  |
Discount rate | 2.81% |

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