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Commitments and Contingent Liabilities (Schedule of Operating Lease ROU Assets and Lease Liabilities) (Details) $ in Millions | Dec. 31, 2019 USD ($)
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Commitments and Contingencies Disclosure [Abstract] |
ROU Asset at Lease Inception | $ 7.3
Accumulated Amortization | 1.4
Current ROU Asset | $ 5.9

X
- Definition Right of use asset at lease inception. + References No definition available. + Details
Name: | msex_RightOfUseAssetAtLeaseInception
Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Accumulated amortization on ROU. + References No definition available. + Details
Name: | msex_RightOfUseAssetsAccumulatedAmortization
Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Right of use of assets current. + References No definition available. + Details
Name: | msex_RightOfUseAssetsCurrent
Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration