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Contingencies and commitments - Summary of aggregate amount of future payment commitments under purchase obligations outstanding (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | $ 19,789 | $ 19,318
Due within 1 year [member] |  |
Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | 2,818 | 2,677
Between 1 and 2 years [member] |  |
Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | 1,664 | 1,711
Between 2 and 3 years [member] |  |
Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | 1,433 | 1,327
Between 3 and 4 years [member] |  |
Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | 1,318 | 1,207
Between 4 and 5 years [member] |  |
Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | 1,062 | 1,040
More than 5 years [member] |  |
Disclosure of future payment commitments under purchase obligations [Line Items] |  |
Aggregate amount of future payment commitments under purchase obligations outstanding | $ 11,494 | $ 11,356

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- Definition Future Payment Commitments Under Purchase Obligations + References No definition available. + Details
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