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Share capital - Rio Tinto plc - Additional information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure Of Classes Of Share Capital [Line Items] |  |  |
ADRs held by the Employee share ownership Trust (ESOT) | 0 | 0 | 0
Share of Rio Tinto plc and American depository receipts [member] |  |  |
Disclosure Of Classes Of Share Capital [Line Items] |  |  |
Value of shares purchased by employee share ownership trusts | $ 38.9 | $ 40.1 | $ 34.1
Rio Tinto plc [member] |  |  |
Disclosure Of Classes Of Share Capital [Line Items] |  |  |
Shares to held in the employee share ownership trusts on behalf of Rio Tinto plc | 835,062 |  |
Rio Tinto plc [member] | American depository receipts [member] |  |  |
Disclosure Of Classes Of Share Capital [Line Items] |  |  |
ADRs held by the Employee share ownership Trust (ESOT) | 38,849 |  |

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