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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 | $ 0.7 us-gaap_AllowanceForDoubtfulAccountsReceivable | $ 0.6 us-gaap_AllowanceForDoubtfulAccountsReceivable |  |  |  |
Bad debt expense | 0.7 us-gaap_ProvisionForDoubtfulAccounts | 0.7 us-gaap_ProvisionForDoubtfulAccounts | 0.7 us-gaap_ProvisionForDoubtfulAccounts |  |  |
Write-offs | 0.6 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs | 0.9 us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs | 0.6 us-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 |  |  |  |  | 4.2 us-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 |  |  |  |  | $ 0.8 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember | $ 3.9 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = msex_TidewaterUtilitiesIncMember / us-gaap_RegulatoryAgencyAxis = msex_DelawarePublicServiceCommissionMember

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