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Classification and measurement of financial instruments - Summary of Reconciliation of Movements in the Fair Values of Financial Instruments Classified as Level 3 (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of fair value measurement of assets and liabilities [line items] |  |  |
Change in fair value | $ 13 | $ 27 | $ (4)
Regent [member] | Level 3 [member] |  |  |
Disclosure of fair value measurement of assets and liabilities [line items] |  |  |
Gain (loss) on contingent purchase consideration | (8) |  |
Exceptional Items [member] |  |  |
Disclosure of fair value measurement of assets and liabilities [line items] |  |  |
Change in fair value | 21 | 38 |
Exceptional Items [member] | Level 3 [member] |  |  |
Disclosure of fair value measurement of assets and liabilities [line items] |  |  |
Change in fair value |  | $ 38 |
Gain (loss) on contingent purchase consideration | 21 |  |
Gain on derivative financial instruments | $ 4 |  |

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- Definition The gain (loss) recognised on designation of a financial instrument, or a proportion of it, as measured at fair value through profit or loss, because a credit derivative is used to manage the credit risk of that financial instrument. [Refer: Credit risk [member]; Derivatives [member]; Financial instruments, class [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 24G -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_24G_b&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The gains (losses) recognised in profit or loss for liabilities measured at fair value that are attributable to the change in unrealised gains or losses relating to those liabilities held at the end of the reporting period. [Refer: At fair value [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 13 -IssueDate 2020-01-01 -Paragraph 93 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=13&code=ifrs-tx-2020-en-r&anchor=para_93_f&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The gains (losses) including exchange differences recognised in profit or loss on the fair value measurement of liabilities. [Refer: At fair value [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 13 -IssueDate 2020-01-01 -Paragraph 93 -Subparagraph e -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=13&code=ifrs-tx-2020-en-r&anchor=para_93_e_i&doctype=Standard -URIDate 2020-01-01 + Details
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