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Operating segments - Reconciliation of net earnings/(losses) to underlying earnings (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Disclosure of operating segments [Line Items] |  |
Post-tax foreign exchange gains | $ 420 |
Post-tax foreign exchange loss | $ 908 |
Discount rate used in net present value of expected future cash flows for the port and rail capacity |  | 2.00%
Post-tax onerous contract charge |  | $ 329
External US Dollar net debt [Member] |  |
Disclosure of operating segments [Line Items] |  |
Post-tax foreign exchange gains |  | 123
Intragroup Balances [Member] |  |
Disclosure of operating segments [Line Items] |  |
Post-tax foreign exchange gains |  | $ 393
Carbone savoie [Member] | Disposal groups [Member] |  |
Disclosure of operating segments [Line Items] |  |
Ownership percentage in subsidiary |  | 100.00%
Bengalla joint venture [Member] | Disposal groups [Member] |  |
Disclosure of operating segments [Line Items] |  |
Percentage of ownership interest exchanged in joint venture |  | 40.00%

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