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Principal accounting policies - Additional information (Detail) | 12 Months Ended
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Dec. 31, 2018 USD ($) Operation | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Retained earnings | $ 27,025,000,000 | $ 23,761,000,000 |
Weighted average incremental borrowing rate applied to lease liabilities | 4.70% |  |
Undiscounted non-cancellable operating lease commitments | $ 1,717,000,000 | 1,845,000,000 |
Non-cancellable operating lease commitments | 500,000 |  |
Non-cancellable operating lease commitments not expected to be charged against EBITDA | 300,000 |  |
Operating lease expenses | $ 787,000,000 | 555,000,000 | $ 541,000,000
Cash-generating units assessed for impairment | 0 |  |
Depreciation or amortisation charges | $ 4,015,000,000 | 4,375,000,000 | $ 4,794,000,000
Onerous contracts provision | $ 324,000,000 |  |
Operations with remaining lives | 10 years |  |
Number of operations | Operation | 13 |  |
Decreasing provision | $ 800,000,000 |  |
Provision for carrying value close-down, restoration and environmental | $ 9,975,000,000 | $ 9,983,000,000 |
Risk free rate decrease | 0.50% |  |
Risk free interest rate on provisions | 2.00% |  |
Intragroup loans | $ 14,000,000,000 |  |
Intangible assets [Member] |  |  |
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Depreciation or amortisation charges | $ 0 |  |
Bottom of range [Member] |  |  |
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Percent of certainty | 50.00% |  |
Assessment period of market prices | three years |  |
Term of impairment review is under taken in relevant functional currency | five years |  |
Risk free interest rate on provisions | 0.50% |  |
Top of range [Member] |  |  |
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Percent of certainty | 90.00% |  |
Assessment period of market prices | five years |  |
IFRS 16 [Member] |  |  |
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Increase in lease liabilities (debt) | $ 1,200,000,000 |  |
Increase in right of use assets/net investments in leases | 1,100,000,000 |  |
Net adjustments to other assets and liabilities | 100,000,000 |  |
Retained earnings | 100,000,000 |  |
Lease liability | 1,300,000,000 |  |
IAS 17 [Member] |  |  |
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Undiscounted non-cancellable operating lease commitments | 1,700,000,000 |  |
IAS 17 [Member] | Entering into significant commitments or contingent liabilities |  |  |
Disclosure of initial application of standards or interpretations [Line Items] |  |  |
Undiscounted non-cancellable operating lease commitments | $ 1,700,000,000 |  |

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