The value of equity securities with the plan.

No definition available.

Name:	msex_CommonStockInPlanValue
Namespace Prefix:	msex_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage employer discretionary contribution to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	msex_DefinedContributionPlanEmployerDiscretionaryContributionPercent
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase (decrease) in annual rate per capita cost of covered healthcare benefits, years four and five.

+ References

No definition available.

+ Details

Name:	msex_IncreaseDecreaseInAnnualRatePerCapitaCostOfCoveredHealthcareBenefitsYearsFourAndFive
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase (decrease) in annual rate per capita cost of covered healthcare benefits,years one to three

+ References

No definition available.

+ Details

Name:	msex_IncreaseDecreaseInAnnualRatePerCapitaCostOfCoveredHealthcareBenefitsYearsOneToThree
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of employer's matching contributions to a defined contribution plan in a given year.

+ References

No definition available.

+ Details

Name:	msex_MatchingContributions
Namespace Prefix:	msex_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of hours required to participate in contribution plan

+ References

No definition available.

+ Details

Name:	msex_NumberOfHoursRequiredToParticipateInContributionPlan
Namespace Prefix:	msex_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percent of participants contributions matched by company, above one percent of participants compensation.

+ References

No definition available.

+ Details

Name:	msex_PercentOfParticipantsContributionsMatchedByCompanyAboveOnePercentOfParticipantsCompensation
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percent of participants contributions matched by company, below one percent of participants compensation.

+ References

No definition available.

+ Details

Name:	msex_PercentOfParticipantsContributionsMatchedByCompanyBelowOnePercentOfParticipantsCompensation
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold percentage of participants compensation at which matching decreases.

+ References

No definition available.

+ Details

Name:	msex_ThresholdPercentageOfParticipantsCompensationAtWhichMatchingDecreases
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (l)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

+ Details

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

- Definition

Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2417-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2417-114920

+ Details

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

- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

+ Details

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

- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

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

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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

- Definition

Amount of cash outflow for pension and other postretirement benefits. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3536-108585

+ Details

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

- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

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

- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_CollateralAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated benefit obligation		$ 73,100	$ 75,700
Increase (decrease) in annual rate per capita cost of covered healthcare benefits, next year		8.00%
Increase (decrease) in annual rate per capita cost of covered healthcare benefits, 2020-2021		1.00%
Increase (decrease) in annual rate per capita cost of covered healthcare benefits, 2022-2023		0.50%
Annual rate of increase in the per capita cost of covered healthcare benefits		5.00%
Number of hours required to participate in contribution plan		1,000
Percent of participants contributions matched by company, below one percent of participants compensation		100.00%
Percent of participants contributions matched by company, above one percent of participants compensation		50.00%
Threshold percentage of participants compensation at which matching decreases		1.00%
Matching contributions, percent		6.00%
Matching contributions		$ 600	600	$ 600
Discretionary contributions, percent		5.00%
Discretionary contributions		$ 600	500	$ 400
Maximum number of shares authorized for grant		300,000
Number of shares authorized for grant, unissued		50,000
Fair value of vested restricted shares		$ 1,300	$ 800
Middlesex [Member] \| Equity Securities [Member]
Allocation of plan assets		1.60%	1.10%
Common stock in plan, value		$ 1,000	$ 800
Pension Benefit Plan [Member]
Allocation of plan assets		1.00%	100.00%
Benfit plan, Cash contributions		$ 3,600
Pension Benefit Plan [Member] \| Equity Securities [Member]
Allocation of plan assets		59.50%	62.80%
Pension Benefit Plan [Member] \| Scenario, Forecast [Member]
Benfit plan, Cash contributions	$ 3,600
Other Benefits Plan [Member]
Allocation of plan assets		1.00%	100.00%
Benfit plan, Cash contributions		$ 1,400
Other Benefits Plan [Member] \| Equity Securities [Member]
Allocation of plan assets		54.70%	57.70%
Other Benefits Plan [Member] \| Scenario, Forecast [Member]
Benfit plan, Cash contributions	$ 1,400