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Exchange rates in US$ - Summary of principal exchange rate used in preparation of financial statements (Detail) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Sterling |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ (in dollars per share) | 1.28 | 1.34 | 1.29
Year-end exchange rate per U.S.$ (in dollars per share) | 1.31 | 1.27 | 1.34
Australian dollar |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ (in dollars per share) | 0.70 | 0.75 | 0.77
Year-end exchange rate per U.S.$ (in dollars per share) | 0.70 | 0.70 | 0.78
Canadian dollar |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ (in dollars per share) | 0.75 | 0.77 | 0.77
Year-end exchange rate per U.S.$ (in dollars per share) | 0.77 | 0.73 | 0.79
Euro |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ (in dollars per share) | 1.12 | 1.18 | 1.13
Year-end exchange rate per U.S.$ (in dollars per share) | 1.12 | 1.14 | 1.20
South African rand |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ (in dollars per share) | 0.069 | 0.076 | 0.075
Year-end exchange rate per U.S.$ (in dollars per share) | 0.071 | 0.069 | 0.081

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- Definition The average exchange rate used by the entity. Exchange rate is the ratio of exchange for two currencies. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The spot exchange rate at the end of the reporting period. Exchange rate is the ratio of exchange for two currencies. Spot exchange rate is the exchange rate for immediate delivery. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard -URIDate 2019-03-27 + Details
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