Note 7 - Derivatives and Hedging Activities - Interest Rate Caps and Swap Agreements Used to Hedge Variable Rate Debt (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Dec. 31, 2018	Jun. 30, 2018
Notional Amount	$ 47,000	$ 47,000
Unrealized Loss	(1,343)	(827)
Liability, Fair Value	$ (1,210)	$ (589)
Interest Rate Swaps Terminating in July 2033 [Member]
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (393)	$ (293)
Interest Rate Swaps Terminating in July 2033 [Member] \| Other Liabilities [Member]
Notional Amount	$ 5,000	$ 5,000
Receive Rate	2.80%	2.05%
Pay Rate	3.38%	3.38%
Liability, Fair Value	$ (393)	$ (293)
Strike Rate
Interest Rate Swaps Terminating in July 2028 [Member]
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (247)	$ (154)
Interest Rate Swaps Terminating in July 2028 [Member] \| Other Liabilities [Member]
Notional Amount	$ 5,000	$ 5,000
Receive Rate	2.80%	2.05%
Pay Rate	3.23%	3.23%
Liability, Fair Value	$ (247)	$ (154)
Strike Rate
Interest Rate Swaps Terminating in July 2023 [Member]
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (51)	$ 15
Interest Rate Swaps Terminating in July 2023 [Member] \| Other Liabilities [Member]
Notional Amount	$ 5,000	$ 5,000
Receive Rate	2.80%	2.05%
Pay Rate	2.77%	2.77%
Liability, Fair Value	$ (51)
Asset, Fair Value		$ 15
Interest Rate Swaps Terminating in September 2029 [Member]
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (212)	$ (81)
Interest Rate Swaps Terminating in September 2029 [Member] \| Other Liabilities [Member]
Notional Amount	$ 6,000	$ 6,000
Receive Rate	5.60%	5.14%
Pay Rate	5.88%	5.88%
Liability, Fair Value	$ (212)	$ (81)
Interest Rate Swaps Terminating in February 2030 [Member]
Notional Amount		$ 10,000
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (346)	$ (140)
Interest Rate Swaps Terminating in February 2030 [Member] \| Other Liabilities [Member]
Notional Amount	$ 10,000
Receive Rate	4.69%	4.23%
Pay Rate	4.98%	4.98%
Liability, Fair Value	$ (346)	$ (140)
Interest Rate Caps Terminating in September 2019 [Member]
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (29)	$ (91)
Interest Rate Caps Terminating in September 2019 [Member] \| Other Assets [Member]
Notional Amount	$ 6,000	$ 6,000
Strike Rate	2.50%	2.50%
Asset, Fair Value	$ 12	$ 15
Interest Rate Caps Terminating in February 2020 [Member]
Index	3 Mo. LIBOR	3 Mo. LIBOR
Unrealized Loss	$ (65)	$ (83)
Interest Rate Caps Terminating in February 2020 [Member] \| Other Assets [Member]
Notional Amount	$ 10,000	$ 10,000
Strike Rate	2.50%	2.50%
Asset, Fair Value	$ 27	$ 49

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageVariableInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeCapInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The underlying refers to the price on the security, commodity, index, or other variable specified by the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeUnderlying
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_UnrealizedGainLossOnDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateSwapsTerminatingInJuly2033Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateSwapsTerminatingInJuly2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateSwapsTerminatingInJuly2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateSwapsTerminatingInSeptember2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateSwapsTerminatingInFebruary2030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateCapsTerminatingInSeptember2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nbn_InterestRateCapsTerminatingInFebruary2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: