Earnings per share - Calculation of Basic and Diluted EPS (Detail) - GBP (£) £ / shares in Units, £ in Millions, shares in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Earnings per share [abstract]
Weighted average shares used in basic EPS calculation	1,224.7	1,249.1	1,250.0
EPS	£ (2.150)	£ 0.250	£ 0.499
Weighted average shares used in diluted EPS calculation	1,237.0	1,257.3	1,260.6
Diluted EPS	£ (2.150)	£ 0.248	£ 0.495
Continuing Operation [member]
Earnings per share [abstract]
Earnings	£ (2,626.5)	£ 269.2	£ 627.9
Weighted average shares used in basic EPS calculation	1,224.7	1,249.1	1,250.0
EPS	£ (2.145)	£ 0.215	£ 0.502
Diluted earnings	£ (2,626.5)	£ 269.2	£ 627.9
Weighted average shares used in diluted EPS calculation	1,224.7	1,257.3	1,260.6
Diluted EPS	£ (2.145)	£ 0.214	£ 0.498
Discontinued operations [member]
Earnings per share [abstract]
Earnings	£ (6.8)	£ 43.2	£ (3.8)
Weighted average shares used in basic EPS calculation	1,224.7	1,249.1	1,250.0
EPS	£ (0.005)	£ 0.035	£ (0.003)
Diluted earnings	£ (6.8)	£ 43.2	£ (3.8)
Weighted average shares used in diluted EPS calculation	1,224.7	1,257.3	1,260.6
Diluted EPS	£ (0.005)	£ 0.034	£ (0.003)
Continued and discontinued operations [member]
Earnings per share [abstract]
Earnings	£ (2,633.3)	£ 312.4	£ 624.1
Weighted average shares used in basic EPS calculation	1,224.7	1,249.1	1,250.0
EPS	£ (2.150)	£ 0.250	£ 0.499
Diluted earnings	£ (2,633.3)	£ 312.4	£ 624.1
Weighted average shares used in diluted EPS calculation	1,224.7	1,257.3	1,260.6
Diluted EPS	£ (2.150)	£ 0.248	£ 0.495

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 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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