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Provisions and other non-current liabilities - Asset retirement obligation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation      
Discount rate for valuation of asset retirement obligation (as a percent) 4.50% 4.50% 4.50%
Inflation rate for valuation of asset retirement obligation (as a percent) 2.00% 2.00% 2.00%
Asset retirement obligations      
Asset Retirement Obligation      
Potential change in discount rate (as a percent) 0.50%    
Increase in asset retirement obligation for decrease in discount rate $ 1,066    
Impact on following years net income for decrease in discount rate (82)    
Changes in the asset retirement obligation      
Balance as of the beginning of the period 12,665 $ 13,314 $ 13,121
Accretion 544 523 513
Revision in estimates (1,107) (558) 685
New obligations 334 375 271
Spending on existing obligations (330) (502) (566)
Currency translation adjustment 448 (395) (676)
Other (314) (92) (34)
Balance as of the end of the period $ 12,240 $ 12,665 $ 13,314