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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