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Employee Benefit Plans (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 N | Dec. 31, 2013 | Dec. 31, 2012 | Jan. 02, 2013
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | $ 61.9 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation | $ 47.5 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation |  |
Increase (decrease) in annual rate per capita cost of covered healthcare benefits, next year | 9.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear |  |  |
Increase (decrease) in annual rate per capita cost of covered healthcare benefits, 2016-2018 | 1.00%msex_IncreaseDecreaseInAnnualRatePerCapitaCostOfCoveredHealthcareBenefitsYearsOneToThree |  |  |
Increase (decrease) in annual rate per capita cost of covered healthcare benefits, 2019-2020 | 0.50%msex_IncreaseDecreaseInAnnualRatePerCapitaCostOfCoveredHealthcareBenefitsYearsFourAndFive |  |  |
Annual rate of increase in the per capita cost of covered healthcare benefits | 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate |  |  |
Number of hours required to participate in contribution plan | 1,000 msex_NumberOfHoursRequiredToParticipateInContributionPlan |  |  |
Percent of participants contributions matched by company, below one percent of participants compensation | 100.00%msex_PercentOfParticipantsContributionsMatchedByCompanyBelowOnePercentOfParticipantsCompensation |  |  |
Percent of participants contributions matched by company, above one percent of participants compensation | 50.00%msex_PercentOfParticipantsContributionsMatchedByCompanyAboveOnePercentOfParticipantsCompensation |  |  |
Threshold percentage of participants compensation at which matching decreases | 1.00%msex_ThresholdPercentageOfParticipantsCompensationAtWhichMatchingDecreases |  |  |
Matching contributions, percent | 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent |  |  |
Matching contributions | 0.6 msex_MatchingContributions | 0.5 msex_MatchingContributions | 0.5 msex_MatchingContributions |
Discretionary contributions, percent | 5.00%msex_DefinedContributionPlanEmployerDiscretionaryContributionPercent |  |  |
Discretionary contributions | 0.3 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount | 0.3 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount | 0.2 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount |
Escrow account maintained for 2008 restricted stock plan | 100,000 msex_EscrowAccountMaintainedFor2008RestrictedStockPlan |  |  |
Maximum number of shares authorized for grant | 300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized |  |  |
Number of shares authorized for grant, unissued | 200,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant |  |  |
Fair value of vested restricted shares | 0.5 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | 0.5 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | 0.3 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue |
Decrease in employee benefit plans liability and related regulatory asset |  |  |  | 10.2 msex_DecreaseInPensionAndOtherPostretirementDefinedBenefitPlansLiabilities
Pension Benefit Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Allocation of plan assets | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |
Benfit plan, Cash contributions | 3.6 us-gaap_PensionAndOtherPostretirementBenefitContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  |
Expected cash contributions next fiscal year | 3.3 us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  |
Other Benefits Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Allocation of plan assets | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember |  |
Benfit plan, Cash contributions | 1.7 us-gaap_PensionAndOtherPostretirementBenefitContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember |  |  |
Expected cash contributions next fiscal year | 1.7 us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember |  |  |
Equity Securities [Member] | Pension Benefit Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Allocation of plan assets | 60.70%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 65.40%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |
Equity Securities [Member] | Other Benefits Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Allocation of plan assets | 49.50%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember | 47.80%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember |  |
Equity Securities [Member] | Middlesex [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Allocation of plan assets | 1.80%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember | 1.80%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember |  |
Common stock in plan, value | $ 0.9 msex_CommonStockInPlanValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember | $ 0.8 msex_CommonStockInPlanValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember |  |

X
- Definition The value of equity securities with the plan. + References No definition available. + Details
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- Definition Refers to decrease in pension and other postretirement defined benefit plans liabilities. + References No definition available. + Details
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- Definition Percentage employer discretionary contribution to a defined contribution plan. + References No definition available. + Details
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- Definition Escrow account maintained for 2008 restricted stock plan + References No definition available. + Details
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- Definition Increase (decrease) in annual rate per capita cost of covered healthcare benefits, years four and five. + References No definition available. + Details
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- Definition Increase (decrease) in annual rate per capita cost of covered healthcare benefits,years one to three + References No definition available. + Details
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- Definition Amount of employer's matching contributions to a defined contribution plan in a given year. + References No definition available. + Details
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- Definition Number of hours required to participate in contribution plan + References No definition available. + Details
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- Definition Percent of participants contributions matched by company, above one percent of participants compensation. + References No definition available. + Details
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- Definition Threshold percentage of participants compensation at which matching decreases. + References No definition available. + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 + Details
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- Definition If aggregate disclosures are presented, the aggregate pension accumulated benefit obligation for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=34716465&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920 + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The ultimate trend rate for health care costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition The amount of cash or cash equivalents contributed during the reporting period by the entity to fund its pension plans and its non-pension postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585 + Details
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- 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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