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.

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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]]

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The percentage of an expected dividend used to calculate the fair value of share options granted.

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The weighted average fair value of share options granted during the period at the measurement date. [Refer: Weighted average [member]]

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Fair value assumptions expected life.

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Weighted average assumptions.

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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, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 GBP (£)
Weighted average assumptions:
Expected life (months)	48 months	48 months	48 months
Expected volatility	24.00%	17.00%	16.00%
Dividend yield	3.50%	2.90%	2.80%
United Kingdom [member]
Weighted average assumptions:
Risk-free interest rate	0.78%	0.57%	0.44%
United Kingdom [member] \| Ordinary shares [member]
Disclosure of terms and conditions of share-based payment arrangement [Line items]
Fair value of options \| £				£ 1.07	£ 1.12	£ 1.35
United States [member]
Weighted average assumptions:
Risk-free interest rate	2.74%	2.05%	1.60%
United States [member] \| American depository receipts [member]
Disclosure of terms and conditions of share-based payment arrangement [Line items]
Fair value of options \| $	$ 8.09	$ 9.40	$ 9.94