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Organization, Summary of Significant Accounting Policies and Recent Developments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended	3 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2014	Jun. 30, 2014	Jun. 30, 2012
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Percent voting interest	50.00%msex_PercentVotingInterest
Percent of operating revenues	88.00%msex_PercentOfOperatingRevenues
Percent of total assets	99.00%msex_PercentOfTotalAssets
Allowance for doubtful accounts	$ 0.7us-gaap_AllowanceForDoubtfulAccountsReceivable	$ 0.6us-gaap_AllowanceForDoubtfulAccountsReceivable
Bad debt expense	0.7us-gaap_ProvisionForDoubtfulAccounts	0.7us-gaap_ProvisionForDoubtfulAccounts	0.7us-gaap_ProvisionForDoubtfulAccounts
Write-offs	0.6us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs	0.9us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs	0.6us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs
Minimum [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Estimated useful lives of non-regulated fixed assets	3 years
Maximum [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Estimated useful lives of non-regulated fixed assets	40 years
Dover Air Force Base [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Term of agreement				50 years
New Jersey Board of Public Utilities [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Approved increase in annual operating revenues					4.2us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / us-gaap_RegulatoryAgencyAxis = msex_NewJerseyBoardOfPublicUtilitiesMember
Delaware Public Service Commission [Member] | Tidewater Utilities Inc. [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Approved increase in annual operating revenues					$ 0.8us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember	$ 3.9us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember

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