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Share-based payments -Summary of performance share plans (Detail)
 12 Months Ended
Dec. 31, 2018
AUD ($)
shares
Dec. 31, 2018
GBP (£)
shares
Dec. 31, 2017
AUD ($)
shares
Dec. 31, 2017
GBP (£)
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 | £   £ 20.47   £ 18.27
Weighted average fair value at grant date, non-vested shares, awarded | £   26.87   24.64
Weighted average fair value at grant date, non-vested shares, forfeited | £   21.45   16.28
Weighted average fair value at grant date, non-vested shares, failed performance conditions | £   14.54   16.25
Weighted average fair value at grant date, non-vested shares, vested | £   21.57   21.51
Weighted average fair value at grant date, non-vested shares at ending balance | £   £ 21.86   £ 20.47
Number of non-vested shares at beginning balance 3,555,274 3,555,274 2,883,053 2,883,053
Number of non-vested shares, awarded 801,047 801,047 1,198,677 1,198,677
Number of non-vested shares, forfeited (188,761) (188,761) (199,435) (199,435)
Number of non-vested shares, failed performance conditions (107,625) (107,625) (161,966) (161,966)
Number of non-vested shares, vested (214,853) (214,853) (165,055) (165,055)
Number of non-vested shares at ending balance 3,845,082 3,845,082 3,555,274 3,555,274
Weighted average share price, Vested awards settled in shares during the year (including dividend shares applied on vesting) | £   £ 42.57   £ 31.04
Weighted average share price, Vested awards settled in cash during the year (including dividend shares applied on vesting) | £   £ 42.40    
Number of vested awards settled in shares during the year (including dividend shares applied on vesting) 248,965 248,965 188,383 188,383
Number of vested awards settled in cash during the year (including dividend shares applied on vesting) 991 991    
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 | $ $ 40.13   $ 37.52  
Weighted average fair value at grant date, non-vested shares, awarded | $ 53.85   45.84  
Weighted average fair value at grant date, non-vested shares, forfeited | $ 43.83   36.59  
Weighted average fair value at grant date, non-vested shares, failed performance conditions | $ 27.67   30.89  
Weighted average fair value at grant date, non-vested shares, vested | $ 40.85   40.79  
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 | $ $ 43.34   $ 40.13  
Number of non-vested shares at beginning balance 1,609,154 1,609,154 1,590,957 1,590,957
Number of non-vested shares, awarded 387,298 387,298 529,776 529,776
Number of non-vested shares, forfeited (36,530) (36,530) (36,716) (36,716)
Number of non-vested shares, failed performance conditions (54,322) (54,322) (113,857) (113,857)
Number of non-vested shares, vested (108,321) (108,321) (84,284) (84,284)
Number of non-vested shares at ending balance 1,797,279 1,797,279 1,609,154 1,609,154
Weighted average share price, Vested awards settled in shares during the year (including dividend shares applied on vesting) | $ $ 82.97   $ 63.09  
Weighted average share price, Vested awards settled in cash during the year (including dividend shares applied on vesting) | $ $ 82.08   $ 62.81  
Number of vested awards settled in shares during the year (including dividend shares applied on vesting) 105,374 105,374 73,311 73,311
Number of vested awards settled in cash during the year (including dividend shares applied on vesting) 9,959 9,959 5,010 5,010