Fair values of financial assets and liabilities- Movements in level 3 financial assets portfolio (Details) - GBP (£) £ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Reconciliation of changes in fair value measurement, assets [abstract]
Gains recognised in other comprehensive income within the revaluation reserve in respect of financial assets at fair value through other comprehensive income	£ 3		£ 5
Level 3
Reconciliation of changes in fair value measurement, assets [abstract]
Balance at beginning of period	16,373	£ 16,179	16,179
Exchange and other adjustments	(20)	136
(Losses) Gains recognised in the income statement within other income	(289)	259
Gains recognised in other comprehensive income within the revaluation reserve in respect of financial assets at fair value through other comprehensive income	43	(67)
Purchases/increases to customer loans	946	853
Sales/repayments of customer loans	(1,592)	(927)
Transfers into the level 3 portfolio	19	114
Transfers out of the level 3 portfolio	(778)	(331)
Balance at end of period	14,702	16,216	16,373
Level 3 \| Financial assets at fair value through profit or loss
Reconciliation of changes in fair value measurement, assets [abstract]
Balance at beginning of period	15,046	14,908	14,908
Exchange and other adjustments	(16)	106
(Losses) Gains recognised in the income statement within other income	(135)	135
Purchases/increases to customer loans	644	851
Sales/repayments of customer loans	(1,520)	(839)
Transfers into the level 3 portfolio	19	73
Transfers out of the level 3 portfolio	(778)	(247)
Balance at end of period	13,260	14,987	15,046
Gains recognised in the income statement, within other income, relating to the change in fair value of those assets held at 30 June 2020	(187)	141
Level 3 \| Financial assets at fair value through other comprehensive income
Reconciliation of changes in fair value measurement, assets [abstract]
Balance at beginning of period	346	408	408
Exchange and other adjustments	(7)	11
Gains recognised in other comprehensive income within the revaluation reserve in respect of financial assets at fair value through other comprehensive income	43	(67)
Purchases/increases to customer loans	0	0
Sales/repayments of customer loans	(8)	(7)
Transfers into the level 3 portfolio	0	0
Transfers out of the level 3 portfolio	0	0
Balance at end of period	374	345	346
Level 3 \| Derivative financial instruments
Reconciliation of changes in fair value measurement, assets [abstract]
Balance at beginning of period	981	863	863
Exchange and other adjustments	3	19
(Losses) Gains recognised in the income statement within other income	(154)	124
Purchases/increases to customer loans	302	2
Sales/repayments of customer loans	(64)	(81)
Transfers into the level 3 portfolio	0	41
Transfers out of the level 3 portfolio	0	(84)
Balance at end of period	1,068	884	£ 981
Gains recognised in the income statement, within other income, relating to the change in fair value of those assets held at 30 June 2020	(156)	132
Level 3 \| Level 3 financial assets carried at fair value
Reconciliation of changes in fair value measurement, assets [abstract]
Gains recognised in the income statement, within other income, relating to the change in fair value of those assets held at 30 June 2020	£ (343)	£ 273

The fair value gains (losses) recognised in other comprehensive income on financial assets reclassified out of the available-for-sale category. [Refer: Financial assets available-for-sale; Other comprehensive income]

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The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

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The gains (losses) including exchange differences recognised in profit or loss on the fair value measurement of assets. [Refer: At fair value [member]]

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The increase (decrease) in financial assets resulting from foreign exchange and other movements. [Refer: Financial assets]

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The amount of other comprehensive income, net of tax, related to gains (losses) in relation to changes in the revaluation surplus. [Refer: Other comprehensive income; Revaluation surplus]

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The increase in the fair value measurement of assets resulting from purchases of those assets. [Refer: At fair value [member]]

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The decrease in the fair value measurement of assets resulting from sales. [Refer: At fair value [member]]

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The amount of transfers of assets into Level 3 of the fair value hierarchy. [Refer: Level 3 of fair value hierarchy [member]]

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The amount of transfers of assets out of Level 3 of the fair value hierarchy. [Refer: Level 3 of fair value hierarchy [member]]

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