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Derivatives and Hedging Activities - Consolidated Statements of Income (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Derivatives and Hedging Activities |  |  |  |  |
Swap contract fees, net of brokerage costs | $ 208,000 | $ 375,000 | $ 820,000 | $ 1,100,000 |
Collateral deposit accounts require to maintain to mitigate counterparty default risk | $ 9,100,000 |  | $ 9,100,000 |  | $ 9,100,000

X
- Definition The amount of collateral deposit accounts require to maintain to mitigate counterparty default risk. + References No definition available. + Details
Name: | nbbk_CollateralDepositAccountsRequireToMaintainToMitigateCounterpartyDefaultRisk
Namespace Prefix: | nbbk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of swap contract fees, net of brokerage costs. + References No definition available. + Details
Name: | nbbk_SwapContractFeesNetOfBrokerageCosts
Namespace Prefix: | nbbk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration