Earnings per share - Calculation of Basic and Diluted EPS (Detail) - GBP (£) £ / shares in Units, £ in Millions, shares in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings per share [abstract]
Weighted average shares used in basic EPS calculation	1,250.0	1,247.8	1,261.1
EPS	£ 0.499	£ 0.852	£ 1.440
Weighted average shares used in diluted EPS calculation	1,260.6	1,261.2	1,275.8
Diluted EPS	£ 0.495	£ 0.843	£ 1.424
Continuing Operation [member]
Earnings per share [abstract]
Earnings	£ 627.9	£ 936.5	£ 1,579.5
Weighted average shares used in basic EPS calculation	1,250.0	1,247.8	1,261.1
EPS	£ 0.502	£ 0.751	£ 1.252
Diluted earnings	£ 627.9	£ 936.5	£ 1,579.5
Weighted average shares used in diluted EPS calculation	1,260.6	1,261.2	1,275.8
Diluted EPS	£ 0.498	£ 0.743	£ 1.238
Discontinued operations [member]
Earnings per share [abstract]
Earnings	£ (3.8)	£ 126.4	£ 237.1
Weighted average shares used in basic EPS calculation	1,250.0	1,247.8	1,261.1
EPS	£ (0.003)	£ 0.101	£ 0.188
Diluted earnings	£ (3.8)	£ 126.4	£ 237.1
Weighted average shares used in diluted EPS calculation	1,260.6	1,261.2	1,275.8
Diluted EPS	£ (0.003)	£ 0.1	£ 0.186
Continued and discontinued operations [member]
Earnings per share [abstract]
Earnings	£ 624.1	£ 1,062.9	£ 1,816.6
Weighted average shares used in basic EPS calculation	1,250.0	1,247.8	1,261.1
EPS	£ 0.499	£ 0.852	£ 1.44
Diluted earnings	£ 624.1	£ 1,062.9	£ 1,816.6
Weighted average shares used in diluted EPS calculation	1,260.6	1,261.2	1,275.8
Diluted EPS	£ 0.495	£ 0.843	£ 1.424

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 profit (loss) attributable to ordinary equity holders of the parent entity. [Refer: Profit (loss)]

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The profit (loss) attributable to ordinary equity holders of the parent entity, adjusted for the effects of all dilutive potential ordinary shares. [Refer: Profit (loss)]

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The number of ordinary shares outstanding at the beginning of the period, adjusted by the number of ordinary shares bought back or issued during the period multiplied by a time-weighting factor.

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