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Financial instruments and risk management - Commodity price risk - Additional information (Detail) - Commodity price risk [Member] lb in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) lb | Dec. 31, 2016 USD ($) lb
Disclosure Of Financial Instruments [Line Items] |  |
Description of hedging strategy | Rio Tinto’s exposure to commodity prices is diversified by virtue of its broad commodity base and the Group does not generally consider commodity price hedging would provide a long-term benefit to shareholders. The Group may hedge certain commitments with some of its customers or suppliers. |
Copper [member] |  |
Disclosure Of Financial Instruments [Line Items] |  |
Quantity of unsettled copper sales | lb | 250 | 235
Provisionally priced unsettled copper sales per pound | $ 3.04 | $ 2.50
Increase (Decrease) in net earnings due to price change | $ 41,000,000 | 36,000,000
Aluminium [Member] |  |
Disclosure Of Financial Instruments [Line Items] |  |
Percentage change in market price on commodity derivatives | 10.00% |
Reduction in net earnings | $ 221,000,000 | 144,000,000
Increase in net earnings | $ 212,000,000 | $ 139,000,000

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