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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
United Kingdom, Pounds |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ | 1.29 | 1.36 | 1.53
Year-end exchange rate per U.S.$ | 1.34 | 1.22 | 1.48
Australian Dollars [Member] |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ | 0.77 | 0.74 | 0.75
Year-end exchange rate per U.S.$ | 0.78 | 0.72 | 0.73
Canada, Dollars [Member] |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ | 0.77 | 0.76 | 0.78
Year-end exchange rate per U.S.$ | 0.79 | 0.74 | 0.72
Euro Member Countries, Euro |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ | 1.13 | 1.11 | 1.11
Year-end exchange rate per U.S.$ | 1.20 | 1.05 | 1.09
South Africa, Rand |  |  |
Disclosure Of Foreign Exchange Rates [Line Items] |  |  |
Full-year average exchange rate per U.S.$ | 0.075 | 0.068 | 0.079
Year-end exchange rate per U.S.$ | 0.081 | 0.073 | 0.064

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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 2016-01-13 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_112_c&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_112_c&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Disclosure of foreign exchange rates. + References No definition available. + Details
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- Details
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