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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, 2016 USD ($)
Pension Benefit Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial Loss | $ 1,566
Prior Service Credit |
Other Benefits Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial Loss | 1,781
Prior Service Credit | $ (1,728)

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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
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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
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Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |