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Commitments and Contingent Liabilities (Schedule of Operating Lease ROU Assets and Lease Liabilities) (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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Commitments and Contingencies Disclosure [Abstract] |  |
ROU Asset at Lease Inception | $ 7.3 | $ 7.3
Accumulated Amortization | (2.3) | (2.1)
Current ROU Asset | $ 5.0 | $ 5.2

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