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Rate and Regulatory Matters (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 3 Months Ended
Mar. 31, 2015
May 31, 2014
Mar. 31, 2015
New Jersey Board of Public Utilities [Member]      
Regulatory Liabilities [Line Items]      
Approved increase in annual operating revenues $ 9.5us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ us-gaap_RegulatoryAgencyAxis
= msex_NewJerseyBoardOfPublicUtilitiesMember		    
Originally filed increase in annual operating revenue   3.6us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
/ us-gaap_RegulatoryAgencyAxis
= msex_NewJerseyBoardOfPublicUtilitiesMember		  
Projected annual revenue     0.3msex_ProjectedAnnualRevenue
/ us-gaap_RegulatoryAgencyAxis
= msex_NewJerseyBoardOfPublicUtilitiesMember
Delware Public Service Commission [Member] | Tidewater [Member]      
Regulatory Liabilities [Line Items]      
Projected annual revenue     $ 0.1msex_ProjectedAnnualRevenue
/ dei_LegalEntityAxis
= msex_TidewaterUtilitiesIncMember
/ us-gaap_RegulatoryAgencyAxis
= msex_DelwarePublicServiceCommissionMember