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Commitments and Contingent Liabilities (Schedule of Operating Lease ROU Assets and Lease Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
ROU Asset at Lease Inception	$ 7.3	$ 7.3
Accumulated Amortization	(1.7)	(1.4)
Current ROU Asset	$ 5.6	$ 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