Pensions - History of defined benefit schemes (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pensions
Fair value of plan assets	£ 37,111	£ 37,598	£ 57,787	£ 57,249	£ 51,925
Present value of plan obligations	(29,592)	(27,601)	(46,808)	(48,864)	(44,115)
Net surplus	7,519	9,997	10,979	8,385	7,810
Actual return on plan assets	£ 659	£ (19,285)	£ 1,667	£ 7,064	£ 4,930
Actual return on plan assets - %	1.80%	(33.40%)	2.90%	13.60%	10.10%
Plan liabilities
Pensions
Experience (losses)/gains	£ (1,599)	£ (2,137)	£ 237	£ 455	£ 279
Plan assets
Pensions
Experience (losses)/gains	(1,182)	(20,326)	872	6,027	3,556
Main Pension Scheme
Pensions
Fair value of plan assets	33,638	34,016	52,021	51,323	46,555
Present value of plan obligations	(26,534)	(24,733)	(42,020)	(43,870)	(39,669)
Net surplus	7,104	9,283	10,001	7,453	6,886
Actual return on plan assets	£ 634	£ (17,248)	£ 1,554	£ 6,422	£ 4,266
Actual return on plan assets - %	1.90%	(33.20%)	3.00%	13.80%	9.70%
Main Pension Scheme \| Plan liabilities
Pensions
Experience (losses)/gains	£ (1,531)	£ (2,053)	£ 241	£ 427	£ 275
Main Pension Scheme \| Plan assets
Pensions
Experience (losses)/gains	£ (1,042)	£ (18,180)	£ 841	£ 5,486	£ 3,021

The amount of other comprehensive income, before tax, resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Other comprehensive income, before tax, gains (losses) on remeasurements of defined benefit plans] [Contrast: Decrease (increase) in net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
-Paragraph 135
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The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
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The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

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- Definition

The amount of other comprehensive income, before tax, resulting from the return on plan assets, excluding amounts included in interest expense (income) arising from defined benefit plans. The return on plan assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Other comprehensive income; Defined benefit plans [member]; Plan assets [member]; Other comprehensive income, before tax, gains (losses) on remeasurements of defined benefit plans] [Contrast: Decrease (increase) in net defined benefit liability (asset) resulting from return on plan assets excluding interest income or expense]

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The fair value of any plan assets, less the present value of the defined benefit obligation. [Refer: Plan assets [member]]

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Percentage of actual return on plan assets.

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