Note 4 - Loans, Allowance for Loan Losses and Credit Quality - Loans Acquired Accounted for Under ASC 310-30 (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Contractually required payments receivable	$ 60,028	$ 49,408	$ 121,568	$ 55,320
Nonaccretable difference	(875)	(1,667)	(1,049)	(1,824)
Cash flows expected to be collected	59,153	47,741	120,519	53,496
Accretable yield	(9,819)	(12,939)	(36,382)	(15,043)
Fair value of loans acquired	$ 49,334	$ 34,802	$ 84,137	$ 38,453

Represents the amount of contractually required payments receivable at the acquisition of acquired loan not accounted for as debt securities with the evidence of deterioration of credit quality.

No definition available.

Name:	nbn_CertainLoanAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition
Namespace Prefix:	nbn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration