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Rate and Regulatory Matters (Narrative) (Details) (USD $) | 3 Months Ended | 0 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 0 Months Ended | 1 Months Ended | 0 Months Ended |
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Jun. 30, 2014 | Nov. 08, 2013 | Jun. 30, 2014 | May 31, 2014 | Jul. 31, 2012 | Dec. 31, 2014 | Mar. 31, 2013 | Aug. 31, 2012 | Jul. 02, 2014 | Apr. 19, 2014 | Nov. 25, 2013 | Jun. 30, 2012 | Oct. 31, 2014 | Oct. 31, 2013 N | Jun. 30, 2011 | Mar. 03, 2012 | Dec. 31, 2013
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liability - Cost of Utility Plant Removal |  |  |  |  |  | $ 10,273,000 us-gaap_RegulatoryLiabilityNoncurrent |  |  |  |  |  |  |  |  |  |  | $ 9,639,000 us-gaap_RegulatoryLiabilityNoncurrent
Acquisition premiums to be recovered |  |  |  |  |  | 600,000 us-gaap_OtherDeferredCostsNet |  |  |  |  |  |  |  |  |  |  |
New Jersey Board of Public Utilities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved increase in annual operating revenues | 4,200,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Jersey Board of Public Utilities [Member] | Middlesex [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved increase in annual operating revenues |  |  | 4,200,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |
Originally filed increase in annual operating revenue |  | 10,600,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  | 3,600,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |  |
Amended filed increase in annual operating revenue |  | 8,100,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmendedAmount / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Base rate amount |  |  | 208,600,000 msex_BaseRateAmount / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |
Return on equity |  |  | 9.75%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |
Projected annual revenue |  |  |  |  |  | 300,000 msex_ProjectedAnnualRevenue / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |
Increase in base rates revenue per year |  |  |  |  | 8,100,000 msex_BaseRateIncrease / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |
Originally filed base water rate increase |  |  |  |  | 11,300,000 msex_OriginallyFiledBaseWaterRateIncrease / dei_LegalEntityAxis = msex_MiddlesexWaterCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |  |  |
New Jersey Board of Public Utilities [Member] | Pinelands Water and Wastewater [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved increase in annual operating revenues |  |  |  |  |  |  | 200,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_PinelandsCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |
Originally filed increase in annual operating revenue |  |  |  |  |  |  |  | 300,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_PinelandsCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |
Approved increase in annual operating revenue (percentage) |  |  |  |  |  |  | 50.00%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage / dei_LegalEntityAxis = msex_PinelandsCompanyMember / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember |  |  |  |  |  |  |  |  |  |
Delaware Public Service Commission [Member] | Tidewater [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved increase in annual operating revenues | 800,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |  |  |  |  |  |  | 3,900,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |
Originally filed increase in annual operating revenue |  |  |  |  |  |  |  |  |  |  | 3,900,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember | 6,900,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |
Approved increase in annual operating revenue (percentage) |  |  |  |  |  |  |  |  | 6.50%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |  |  |  |
Amended filed increase in annual operating revenue |  |  |  |  |  |  |  |  | 2,500,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmendedAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember | 800,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmendedAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |  |  |
Refund of previously deferred revenues |  |  |  |  |  | 400,000 us-gaap_DeferredRevenueRefundPayments / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |  |  |  |  |  |  |
Projected annual revenue |  |  |  |  |  | 100,000 msex_ProjectedAnnualRevenue / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |  |  |  |  |  |  |
Delaware Public Service Commission [Member] | TESI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved increase in annual operating revenues |  |  |  |  |  |  |  |  |  |  |  | 600,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember | 100,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember | 200,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |
Originally filed increase in annual operating revenue |  |  |  |  |  |  |  |  |  |  |  | 800,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |  |
Approved increase in annual operating revenue (percentage) |  |  |  |  |  |  |  |  |  |  |  |  | 33.50%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |  |
Number of customers |  |  |  |  |  |  |  |  |  |  |  |  |  | 600 msex_NumberOfCustomers / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |
Purchase of the wastewater utility assets |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 msex_PurchaseOfWastewaterUtilityAssets / dei_LegalEntityAxis = msex_TidewaterEnvironmentalServicesMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |  |
Delaware Public Service Commission [Member] | Southern Shores [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Originally filed increase in annual operating revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_SouthernShoresMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember |  |
Pennsylvania Public Utilities Commission [Member] | Twin Lakes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Originally filed increase in annual operating revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TwinLakesMember / us-gaap_RegulatoryAgencyAxis = msex_PennsylvaniaPublicUtilitiesCommissionMember |

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