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Share-based payments -Summary of performance share plans (Detail)
 12 Months Ended
Dec. 31, 2017
GBP (£)
shares
AUD ($)
Dec. 31, 2017
AUD ($)
shares
Dec. 31, 2016
GBP (£)
shares
Dec. 31, 2016
AUD ($)
shares
Rio Tinto Limited [member]        
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]        
Number of transfers from equity to cash settled (276,722) (276,722)    
Performance share plans [member] | Rio Tinto plc [member]        
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]        
Weighted average fair value at grant date, non-vested shares at beginning balance | £ £ 18.27   £ 21.29  
Weighted average fair value at grant date, non-vested shares, awarded | £ 24.64   12.81  
Weighted average fair value at grant date, non-vested shares, forfeited | £ 16.28   17.22  
Weighted average fair value at grant date, non-vested shares, failed performance conditions | £ 16.25   24.61  
Weighted average fair value at grant date, non-vested shares, vested | £ 21.51   24.61  
Weighted average fair value at grant date, non-vested shares at ending balance | £ £ 20.47   £ 18.27  
Number of non-vested shares at beginning balance | £ 2,883,053   2,988,454  
Number of non-vested shares, awarded | £ 1,198,677   1,085,974  
Number of non-vested shares, forfeited | £ (199,435)   (603,871)  
Number of non-vested shares, failed performance conditions | £ (161,966)   (203,491)  
Number of non-vested shares, vested | £ (165,055)   (384,013)  
Number of non-vested shares at ending balance | £ 3,555,274   2,883,053  
Weighted average share price, Vested awards settled in shares during the year (including dividend shares applied on vesting) | £ £ 31.04   £ 19.08  
Weighted average share price, Vested awards settled in cash during the year (including dividend shares applied on vesting) | £     £ 19.00  
Number of vested awards settled in shares during the year (including dividend shares applied on vesting) | shares 188,383 188,383 368,623 368,623
Number of vested awards settled in cash during the year (including dividend shares applied on vesting) | shares     486 486
Performance share plans [member] | Rio Tinto Limited [member]        
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]        
Weighted average fair value at grant date, non-vested shares at beginning balance   $ 37.52   $ 41.13
Weighted average fair value at grant date, non-vested shares, awarded   45.84   28.66
Weighted average fair value at grant date, non-vested shares, forfeited   36.59   36.18
Weighted average fair value at grant date, non-vested shares, failed performance conditions   30.89   44.71
Weighted average fair value at grant date, non-vested shares, vested   40.79   44.71
Weighted average fair value transfers from equity to cash settled   40.11    
Weighted average fair value at grant date, non-vested shares at ending balance   $ 40.13   $ 37.52
Number of non-vested shares at beginning balance   1,590,957   2,048,823
Number of non-vested shares, awarded   529,776   535,418
Number of non-vested shares, forfeited   (36,716)   (526,212)
Number of non-vested shares, failed performance conditions   (113,857)   (161,937)
Number of non-vested shares, vested   (84,284)   (305,135)
Number of non-vested shares at ending balance   1,609,154   1,590,957
Weighted average share price, Vested awards settled in shares during the year (including dividend shares applied on vesting)   $ 63.09   $ 42.99
Weighted average share price, Vested awards settled in cash during the year (including dividend shares applied on vesting)   $ 62.81   $ 43.47
Number of vested awards settled in shares during the year (including dividend shares applied on vesting) | shares 73,311 73,311 284,507 284,507
Number of vested awards settled in cash during the year (including dividend shares applied on vesting) | shares 5,010 5,010 847 847