Earnings Per Share - Summary of Earnings Per Share (Detail) - GBP (£) £ / shares in Units, £ in Millions, shares in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings per share [abstract]
Earnings/(loss) for the year from continuing operations	£ 590	£ 408	£ (2,335)
Non-controlling interest	(2)	(2)	(2)
Earnings/(loss) attributable to equity holders of the company	£ 588	£ 406	£ (2,337)
Weighted average number of shares (millions)	778.1	813.4	814.8
Effect of dilutive share options (millions)	0.6	0.3
Weighted average number of shares (millions) for diluted earnings	778.7	813.7	814.8
Earnings/(loss) per share
Basic	£ 0.756	£ 0.499	£ (2.868)
Diluted	£ 0.755	£ 0.499	£ (2.868)

The weighted average number of ordinary shares outstanding plus the weighted average number of ordinary shares that would be issued on the conversion of all the dilutive potential ordinary shares into ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator) divided by the weighted average number of ordinary shares outstanding during the period (the denominator).

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator), divided by the weighted average number of ordinary shares outstanding during the period (the denominator), both adjusted for the effects of all dilutive potential ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The number of dilutive potential ordinary shares that relate to the assumed exercise of the entity's share options.

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The profit (loss) from continuing and discontinued operations attributable to non-controlling interests. [Refer: Profit (loss); Non-controlling interests]

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The profit (loss) from continuing and discontinued operations attributable to owners of the parent. [Refer: Profit (loss)]

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The profit (loss) from continuing operations. [Refer: Continuing operations [member]; Profit (loss)]

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