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Employee Benefit Plans (Schedule of Amounts Expected to be Amortized from Regulaotry Assets to Net Periodic Benefit Cost) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Pension Benefit Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected amortized actuarial Loss | $ 1,646 msex_ExpectedAmortizedActuarialLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Expected amortized prior service credit |
Other Benefits Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected amortized actuarial Loss | 2,261 msex_ExpectedAmortizedActuarialLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember
Expected amortized prior service credit | $ (1,728)msex_ExpectedAmortizedPriorServiceCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPensionPlansDefinedBenefitMember

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- Definition Expected amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions. + References No definition available. + Details
Name: | msex_ExpectedAmortizedActuarialLoss
Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Expected amount of amortization of the net prior service credits. + References No definition available. + Details
Name: | msex_ExpectedAmortizedPriorServiceCredit
Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_OtherPensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |