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v3.19.3.a.u2

Employee Benefit Plans (Schedule of Amounts Expected to be Amortized from Regulatory Assets to Net Periodic Benefit Cost) (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 USD ($)
Pension Benefit Plan [Member] |
Actuarial Loss | $ 2,059
Other Benefits Plan [Member] |
Actuarial Loss | $ 1,351

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

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