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Significant accounting policies, judgements, estimates and assumptions - Impairment of property, plant and equipment, intangible assets, and goodwill (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
usd_per_bbl
Dec. 31, 2017
USD ($)
usd_per_mmBtu
Dec. 31, 2016
USD ($)
usd_per_bbl
Dec. 31, 2016
USD ($)
usd_per_mmBtu
Dec. 31, 2015
USD ($)
Disclosure of impairment loss and reversal of impairment loss [line items]          
Discount rate used to determine recoverable costs, fair value less costs of disposal 6.00% 6.00% 6.00% 6.00%  
Discount rate used in current estimate of value in use 9.00% 9.00% 9.00% 9.00%  
Premium added to discount rate 2.00% 2.00% 2.00% 2.00%  
Long-term price assumptions used in current measurement of fair value less costs of disposal 75 4 75 4  
Oil and gas properties          
Disclosure of impairment loss and reversal of impairment loss [line items]          
Increase in risk factors used in any single test, that could impact asset carrying amount 10.00% 10.00%      
Increase in asset carrying amount as a result of increase in risk factors in any single test $ 400 $ 400      
Oil and gas properties | Minimum          
Disclosure of impairment loss and reversal of impairment loss [line items]          
Decrease in forecasted cash flow, when risk factors are applied which do not meet the criteria to be treated as proved 10.00% 10.00%      
Oil and gas properties | Maximum          
Disclosure of impairment loss and reversal of impairment loss [line items]          
Decrease in forecasted cash flow, when risk factors are applied which do not meet the criteria to be treated as proved 90.00% 90.00%      
Goodwill          
Disclosure of impairment loss and reversal of impairment loss [line items]          
Intangible assets and goodwill $ 11,551 $ 11,551 $ 11,194 $ 11,194 $ 11,627