Net investment income/(expense) (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Gains (losses) on financial instruments [abstract]
Net gains/(losses) from financial assets mandatorily at fair value	£ 133	£ 19	£ (116)
Net gains/(losses) from disposal of debt instruments at fair value through other comprehensive income	102	(68)	248
Net (losses)/gains from disposal of financial assets and liabilities measured at amortised cost	(9)	(66)	22
Net losses on other investments	(114)	(208)	(234)
Net investment income/(expense)	£ 112	£ (323)	£ (80)

The amount of income or (expense) arising from investments.

No definition available.

Name:	bbplc_NetInvestmentIncome
Namespace Prefix:	bbplc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of investment income that the entity does not separately disclose in the same statement or note.

+ References

No definition available.

+ Details

Name:	bbplc_OtherInvestmentIncome
Namespace Prefix:	bbplc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cumulative gain (loss) on disposal of investments in equity instruments that the entity has designated at fair value through other comprehensive income. [Refer: At fair value [member]; Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2023-01-01
-Paragraph 11B
-Subparagraph c
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2023-en-r&anchor=para_11B_c&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_CumulativeGainLossOnDisposalOfInvestmentsInEquityInstrumentsDesignatedAsMeasuredAtFairValueThroughOtherComprehensiveIncome
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The gain (loss) arising from the derecognition of financial assets measured at amortised cost. [Refer: Financial assets at amortised cost]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2023-01-01
-Paragraph 82
-Subparagraph aa
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2023-en-r&anchor=para_82_aa&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_GainLossArisingFromDerecognitionOfFinancialAssetsMeasuredAtAmortisedCost
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The gains (losses) on financial assets mandatorily measured at fair value through profit or loss in accordance with IFRS 9. [Refer: Financial assets at fair value through profit or loss, mandatorily measured at fair value; Gains (losses) on financial assets at fair value through profit or loss; Financial liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2023-01-01
-Paragraph 20
-Subparagraph a
-Clause i
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2023-en-r&anchor=para_20_a_i&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_GainsLossesOnFinancialAssetsAtFairValueThroughProfitOrLossMandatorilyMeasuredAtFairValue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	ifrs-full_GainsLossesOnFinancialInstrumentsAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration