Represents the amount of annual fee for asset management and consulting services.

No definition available.

Name:	nen_AssetManagementAnnualFees
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Revenue derived from construction, architectural services and supervision of capital projects provided under contracts or arrangements with clients.

+ References

No definition available.

+ Details

Name:	nen_ConstructionArchitecturalServicesAndSupervisionOfCapitalProjects
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the amount aggregate cost of construction, architectural services and supervision of capital projects during the reporting period.

+ References

No definition available.

+ Details

Name:	nen_CostOfServicesArchitecturalServicesAndSupervisionOfProjects
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the expenses incurred for attendance and participation in advisory committee meeting.

+ References

No definition available.

+ Details

Name:	nen_ExpensesIncurredForAttendanceAndParticipationInAdvisoryCommitteeMeeting
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the expenses incurred for attendance and participation in Advisory Committee meeting.

+ References

No definition available.

+ Details

Name:	nen_ExpensesIncurredForAttendanceAndParticipationInAuditCommitteeMeeting
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of limited partnerships and limited liability companies in which the entity has made investments which are accounted for using the equity method of consolidation.

+ References

No definition available.

+ Details

Name:	nen_InvestmentsInNumberOfLimitedPartnershipsAndLimitedLiabilityCompanies
Namespace Prefix:	nen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the amount of management fee incurred for payment to a management company during the reporting period.

+ References

No definition available.

+ Details

Name:	nen_ManagementFees
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of Advisory Committee meeting held during the reporting period.

+ References

No definition available.

+ Details

Name:	nen_NumberOfAdvisoryCommitteeMeetingHeld
Namespace Prefix:	nen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of Audit Committee meeting held during the period.

+ References

No definition available.

+ Details

Name:	nen_NumberOfAuditCommitteeMeetingHeld
Namespace Prefix:	nen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of employees, excluding President, having ownership interest in the investment properties.

+ References

No definition available.

+ Details

Name:	nen_NumberOfEmployeesHavingOwnershipInterest
Namespace Prefix:	nen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of revenue derived from managing real estate properties and from providing other services which may include construction, architectural services, supervision of capital projects, repairs and maintenance, and administrative services.

+ References

No definition available.

+ Details

Name:	nen_PropertyManagementFeeRevenueAndOtherServiceRevenue
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Revenue from a related party for administrative services.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyAdmistrativeServicesRevenue
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Revenue from a related party for maintenace services.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyMaintenanceRevenue
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee by related party.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyOwnershipInterest
Namespace Prefix:	nen_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the amount of expense charged to entity for bookkeeping and accounting services under terms of agreement with related parties, included within administrative costs.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyTransactionBookkeepingAndAccountingServicesFee
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the management fee expressed as a percentage of rental revenues from properties owned.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyTransactionManagementFeeAsPercentageOfPropertyRentalRevenueAndLaundryIncome
Namespace Prefix:	nen_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of employees who perform accounting and bookkeeping functions.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyTransactionNumberOfAccountingStaff
Namespace Prefix:	nen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount reimbursed to related party for transfer of employee payroll.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyTransactionReimbursementToRelatedPartyForEmployeePayroll
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amounts charged by related party for services provided during the financial reporting period, excluding management fee. Services may include legal, accounting, administrative, repairs and maintenance, construction, rental and architectural services and supervision of capital projects.

+ References

No definition available.

+ Details

Name:	nen_RelatedPartyTransactionServicesVariousProfessionalServicesExcludingManagementFee
Namespace Prefix:	nen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate costs of keeping the property in good condition but that do not appreciably prolong the life or increase the value of the property.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Name:	us-gaap_CostOfPropertyRepairsAndMaintenance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

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

- Definition

The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.4)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Name:	us-gaap_GeneralAndAdministrativeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (210.5-03(11))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.9)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

Name:	us-gaap_InterestExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyManagementFeePercentFee
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of revenue derived from managing real estate properties.

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=nen_MajorityOfPartnershipPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=nen_HamiltonLinewtLLCCommercial1031ExchangeNewtonMassachusettsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis=nen_InvestmentPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=nen_HamiltonCompanyIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nen_HamiltonCompanyIncRelatedPartyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nen_InvestmentPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nen_InvestmentPropertiesExcludingDexterParkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nen_HamiltonParkTowersLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nen_PresidentOfManagementCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nen_HaroldBrownMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=nen_ResidentialAndMixedUseRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstatePropertiesAxis=us-gaap_PartiallyOwnedPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

RELATED PARTY TRANSACTIONS (Details)	12 Months Ended
RELATED PARTY TRANSACTIONS (Details)	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
RELATED PARTY TRANSACTIONS
Management fee	$ 2,452,814	$ 2,409,151	$ 2,326,225
Repairs and maintenance	8,781,789	9,191,561	9,187,714
Administrative expense	2,209,780	2,495,272	2,204,923
Employer contributions in 401K plan	$ 45,000	45,000
Number of advisory committee meetings \| item	7
Expenses incurred for advisory committee meeting	$ 35,250
Number of audit committee meetings held \| item	4
Expenses incurred for audit committee meeting	$ 32,000
Interest expense	$ 13,705,415	12,201,966	12,389,680
Majority of Partnership Properties, Excluding Linewt
RELATED PARTY TRANSACTIONS
Management fee as percentage of gross receipts rental revenue	4.00%
Hamilton Linewt LLC Commercial-1031 Exchange Newton, Massachusetts
RELATED PARTY TRANSACTIONS
Management fee as percentage of gross receipts rental revenue	3.00%
Investment Properties
RELATED PARTY TRANSACTIONS
Management fee	$ 580,029	664,341	652,200
Repairs and maintenance	2,441,880	2,511,741	2,666,148
Administrative expense	$ 327,697	377,298	360,792
Number of limited partnerships and limited liability companies in which the entity has invested \| item	7
Interest expense	$ 6,742,727	6,915,193	6,866,175
Investment Properties \| Minimum
RELATED PARTY TRANSACTIONS
Ownership interest (as a percent)	40.00%
Investment Properties \| Maximum
RELATED PARTY TRANSACTIONS
Ownership interest (as a percent)	50.00%
Hamilton Company/Management Company
RELATED PARTY TRANSACTIONS
Employer contributions in 401K plan	$ 45,000	45,000	0	$ 0
Number of accounting staff of related party providing bookkeeping and accounting functions \| item	14
Number of employees having ownership interest in the investment properties \| item	5
Hamilton Company/Management Company, Related Party
RELATED PARTY TRANSACTIONS
Management fee	$ 2,453,000	2,409,000	2,326,000
Related Party Transaction, Services, Excluding Management Fee	1,080,000	1,702,000	1,380,000
Repairs and maintenance	191,000
Administrative expense	221,000
Construction, architectural services and supervision of capital projects	668,000
Reimbursement to related party for payroll transfers	3,535,000	$ 3,384,000	3,347,000
Fees for accounting and bookkeeping services			$ 125,000	$ 125,000	$ 125,000
Investment Properties \| Hamilton Company/Management Company
RELATED PARTY TRANSACTIONS
Property Management Fee Revenue And Other Service Revenue	774,000
Amount received for management fees	580,000
Amount received for construction, architectural services and supervision of capital projects	145,000
Amount received for maintenance services	34,000
Amount received for administrative services	$ 14,000
Number of employees having ownership interest in the investment properties \| item	5
Investment Properties, Excluding Dexter Park \| Hamilton Company/Management Company
RELATED PARTY TRANSACTIONS
Property management fee received (as a percent)	4.00%
Dexter Park \| Hamilton Company/Management Company
RELATED PARTY TRANSACTIONS
Property management fee received (as a percent)	2.00%
President of Management Company
RELATED PARTY TRANSACTIONS
Annual fee for asset management consulting services	$ 37,500
Harold Brown \| Investment Properties \| Minimum
RELATED PARTY TRANSACTIONS
Ownership interest of related party (as a percent)	47.60%
Harold Brown \| Investment Properties \| Maximum
RELATED PARTY TRANSACTIONS
Ownership interest of related party (as a percent)	59.00%	59.00%
Residential and mixed-use properties \| Partially owned properties \| Minimum
RELATED PARTY TRANSACTIONS
Ownership interest (as a percent)	40.00%
Residential and mixed-use properties \| Partially owned properties \| Investment Properties \| Minimum
RELATED PARTY TRANSACTIONS
Ownership interest (as a percent)	40.00%
Residential and mixed-use properties \| Partially owned properties \| Investment Properties \| Maximum
RELATED PARTY TRANSACTIONS
Ownership interest (as a percent)	50.00%