Retirement Benefit and Other Post-retirement Obligations - Summary of Gains (Losses) Recognised in Other Comprehensive Income (Detail) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disclosure of defined benefit plans gain loss recognised [line items]
Gains (losses) recognised for benefit plans	£ (85)	£ 54	£ 149
Defined benefit plan [member]
Disclosure of defined benefit plans gain loss recognised [line items]
Gains (losses) recognised for benefit plans	(86)	44	145
PRMB [member]
Disclosure of defined benefit plans gain loss recognised [line items]
Gains (losses) recognised for benefit plans	£ 1	£ 10	£ 4

The amount of other comprehensive income, before tax, related to gains (losses) on remeasurements of defined benefit plans, which comprise actuarial gains and losses; the return on plan assets, excluding amounts included in net interest on the net defined benefit liability (asset); and any change in the effect of the asset ceiling, excluding amounts included in net interest on the net defined benefit liability (asset). [Refer: Other comprehensive income; Defined benefit plans [member]; Plan assets [member]; Net defined benefit liability (asset)] [Contrast: Decrease (increase) in net defined benefit liability (asset) resulting from gain (loss) on remeasurement in other comprehensive income]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration