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Financial instruments and risk management - Capital and liquidity risk management - Additional information (Detail) - USD ($)
$ in Millions
 4 Months Ended 12 Months Ended
Apr. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Instruments [Line Items]        
Net debt   $ (255) $ 3,845 $ 9,587
Net cash   11,821 13,884 8,465
Buy-back of bonds $ 1,900   2,500 $ 7,500
Borrowings   $ 12,752 15,176  
Percentage of change in spot price   15.00%    
Level 1 [member]        
Disclosure Of Financial Instruments [Line Items]        
Borrowings   $ 7,500 9,600  
Borrowings at fair value   8,300 10,900  
Level 3 [member]        
Disclosure Of Financial Instruments [Line Items]        
Borrowings   4,200 4,100  
Borrowings at fair value   $ 4,600 4,700  
Percentage of increase in copper price   10.00%    
Period of London metal exchange curve   ten-year    
Decrease in carrying value of forward contract due to long-term metal pricing assumptions   $ 22 41  
Increase in carrying value of forward contract due to long-term metal pricing assumptions   $ 14 22  
Percentage of change in coal spot price   15.00%    
Increase in carrying value of forward contract due to coal spot price assumptions   $ 181    
Decrease in carrying value of forward contract due to decrease in spot price   95    
Capital and liquidity risk management [member]        
Disclosure Of Financial Instruments [Line Items]        
Net debt   (255) $ 3,845  
Net cash   $ 300    
Net debt to total capital percentage   (1.00%) 7.00%  
Description of interest coverage on debt   22 times 14 times  
Revolving credit facilities period   1 year    
Borrowing maturity   November 2022    
Description of rating agencies used   Standard & Poor’s and Moody’s investor services    
Capital and liquidity risk management [member] | Rio Tinto Finance (USA) plc and Rio Tinto Finance (USA) Limited [member]        
Disclosure Of Financial Instruments [Line Items]        
Buy-back of bonds   $ 1,900 $ 2,500  
Aggregate cash outflow before fees   2,000 $ 2,700  
Capital and liquidity risk management [member] | November Two Thousand And Twenty One        
Disclosure Of Financial Instruments [Line Items]        
Revolving credit facilities   1,900    
Capital and liquidity risk management [member] | November Two Thousand And Twenty Two        
Disclosure Of Financial Instruments [Line Items]        
Revolving credit facilities   $ 5,600    
Bottom of range [Member] | Capital and liquidity risk management [member]        
Disclosure Of Financial Instruments [Line Items]        
Percentage of underlying earnings   40.00%    
Top of range [Member] | Capital and liquidity risk management [member]        
Disclosure Of Financial Instruments [Line Items]        
Percentage of underlying earnings   60.00%