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Authorised and issued share capital - Weighted Average Fair Value of Options Granted and Weighted Average Assumptions (Detail) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2019 GBP (£) | Dec. 31, 2018 GBP (£) | Dec. 31, 2017 GBP (£) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Weighted average assumptions: |  |  |  |  |  |
Expected life (months) | 48 months | 48 months | 48 months |  |  |
Expected volatility | 24.00% | 24.00% | 17.00% |  |  |
Dividend yield | 3.80% | 3.50% | 2.90% |  |  |
United Kingdom [member] |  |  |  |  |  |
Weighted average assumptions: |  |  |  |  |  |
Risk-free interest rate | 0.57% | 0.78% | 0.57% |  |  |
United Kingdom [member] | Ordinary shares [member] |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line items] |  |  |  |  |  |
Fair value of options | £ | £ 1.17 | £ 1.07 | £ 1.12 |  |  |
United States [member] |  |  |  |  |  |
Weighted average assumptions: |  |  |  |  |  |
Risk-free interest rate | 1.61% | 2.74% | 2.05% |  |  |
United States [member] | American depository receipts [member] |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line items] |  |  |  |  |  |
Fair value of options | $ |  |  |  | $ 8.49 | $ 8.09 | $ 9.40

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- Definition The expected volatility of the share price used to calculate the fair value of the share options granted. Expected volatility is a measure of the amount by which a price is expected to fluctuate during a period. The measure of volatility used in option pricing models is the annualised standard deviation of the continuously compounded rates of return on the share over a period of time. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 2 -IssueDate 2019-01-01 -Paragraph 47 -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_47_a_i&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The implied yield currently available on zero-coupon government issues of the country in whose currency the exercise price for share options granted is expressed, with a remaining term equal to the expected term of the option being valued (based on the option's remaining contractual life and taking into account the effects of expected early exercise). [Refer: Government [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 2 -IssueDate 2019-01-01 -Paragraph 47 -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_47_a_i&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The percentage of an expected dividend used to calculate the fair value of share options granted. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 2 -IssueDate 2019-01-01 -Paragraph 47 -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_47_a_i&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The weighted average fair value of share options granted during the period at the measurement date. [Refer: Weighted average [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 2 -IssueDate 2019-01-01 -Paragraph 47 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_47_a&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition Weighted average assumptions. + References No definition available. + Details
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