Subordinated liabilities - Narrative (Details) - GBP (£) £ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2026	Dec. 31, 2025
Disclosure of detailed information about borrowings [line items]
Proceeds from issue of subordinated liabilities	£ 4,465	£ 9,808
Redemption of subordinated debt	6,239	£ 5,665
Barclays Bank PLC loans issued intra-group Barclays PLC \| Barclays PLC
Disclosure of detailed information about borrowings [line items]
Proceeds from issue of subordinated liabilities	4,215
Redemption of subordinated debt	4,219
6.174% Fixed Rate Resetting Subordinated Callable Notes (GBP) \| Fixed interest rate \| Barclays PLC
Disclosure of detailed information about borrowings [line items]
Proceeds from issue of subordinated liabilities	£ 250
Interest rate	6.174%
1.125% Fixed Rate Resetting Subordinated Callable Notes (EUR) \| Fixed interest rate \| Barclays PLC
Disclosure of detailed information about borrowings [line items]
Interest rate	1.125%
Redemption of subordinated debt	£ 862
5.200% Fixed Rate Resetting Subordinated Callable Notes (USD) \| Fixed interest rate \| Barclays PLC
Disclosure of detailed information about borrowings [line items]
Interest rate	5.20%
Redemption of subordinated debt	£ 1,003
Floating Rate Notes (ZAR) \| Floating interest rate \| Barclays Bank PLC Subsidiary
Disclosure of detailed information about borrowings [line items]
Redemption of subordinated debt	£ 155

The interest rate on borrowings. [Refer: Borrowings]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2025-01-01
-Paragraph 7
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2025-en-r&anchor=para_7&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_BorrowingsInterestRate
Namespace Prefix:	ifrs-full_
Data Type:	dtr-2022:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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

- Definition

The cash inflow from the issuing of subordinated liabilities. [Refer: Subordinated liabilities]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2025-01-01
-Paragraph 17
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2025-en-r&anchor=para_17&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_ProceedsFromIssueOfSubordinatedLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for repayments of subordinated liabilities. [Refer: Subordinated liabilities]

+ References

+ Details

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

- Details

Name:	ifrs-full_BorrowingsByNameAxis=bbplc_SubordinatedLiabilitiesLoansIssuedIntraGroupToUltimateParentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=ifrs-full_ParentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=bbplc_A6.174FixedRateResettingSubordinatedCallableNotesGBPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=ifrs-full_FixedInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=bbplc_A1.125FixedRateResettingSubordinatedCallableNotesEURMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=bbplc_A5.200FixedRateResettingSubordinatedCallableNotesUSDMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=bbplc_FloatingRateNotesZARMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=ifrs-full_FloatingInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=bbplc_BarclaysBankPLCSubsidiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: