Share-based payments - Summary of Information about Awards Granted (Detail)	12 Months Ended
Dec. 31, 2020 yr £ / shares	Dec. 31, 2019 yr £ / shares	Dec. 31, 2018 yr £ / shares
Annual Performance Plan [member]
Information regarding option pricing models and assumptions used for grants during the year [line items]
Weighted average share price (pence) \| £ / shares	£ 37.71	£ 45.97	£ 46.45
Term (years) \| yr	3.0	3.0	3.0
Long Term Incentive Plan [member]
Information regarding option pricing models and assumptions used for grants during the year [line items]
Weighted average share price (pence) \| £ / shares	£ 34.50	£ 48.50	£ 47.74
Expected dividend yield	1.48%	2.16%	2.27%
Risk-free interest rate	0.02%	0.72%	0.84%
Volatility	33.00%	19.00%	25.00%
Term (years) \| yr	3.0	3.0	3.0

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2020-01-01
-Paragraph 47
-Subparagraph a
-Clause i
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-URIDate 2020-01-01

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- Definition

The option life of share options granted.

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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]]

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- Definition

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

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- Definition

The weighted average share price used as input to the option pricing model to calculate the fair value of share options granted. [Refer: Option pricing model [member]; Weighted average [member]]

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- Definition

fair value of share award using monte carlo pricing model and weighted average assumptions.

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No definition available.

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