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Short-term Borrowings (Narrative) (Details)	6 Months Ended
	Jun. 30, 2020
Line of Credit Facility [Line Items]
Weighted average interest rate on outstanding borrowings	1.21%
CoBank [Member]
Line of Credit Facility [Line Items]
Renewal term of line of credit	24 months

X

- Definition

Renewal term of line of credit.

+ References

No definition available.

+ Details

Name:	msex_RenewalTermOfLineOfCredit
Namespace Prefix:	msex_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- 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

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=msex_CoBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: