Commitments and Contingencies - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Commitment and Contingencies
Provision for (release of) credit losses - unfunded commitments	$ 355,000	$ (2,374,000)	$ (517,000)	$ (2,597,000)
Unfunded loan commitment
Commitment and Contingencies
Provision for (release of) credit losses - unfunded commitments			2,700,000		$ 3,200,000
Standby letters
Commitment and Contingencies
Loss contingency accrual	$ 5,600,000		$ 5,600,000		$ 6,000,000

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

NB Bancorp, Inc. 10-Q