Short-term Borrowings (Schedule of Line of Credit) (Details) $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($)
Line of Credit Facility [Line Items]
Outstanding	$ 13.0
Available	97.0
Maximum	110.0
Bank of America [Member]
Line of Credit Facility [Line Items]
Outstanding
Available	30.0
Maximum	$ 30.0
Credit Type	Uncommitted
Renewal Date	Jan. 27, 2022
PNC Bank [Member]
Line of Credit Facility [Line Items]
Outstanding	$ 10.0
Available	58.0
Maximum	$ 68.0
Credit Type	Committed
Renewal Date	Jan. 31, 2023
CoBank [Member]
Line of Credit Facility [Line Items]
Outstanding	$ 3.0
Available	9.0
Maximum	$ 12.0
Credit Type	Committed
Renewal Date	Nov. 30, 2023

Renewal Date of line of credit.

No definition available.

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Period Type:	duration

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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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Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Information about borrowings which initially required repayment in less than twelve months (or normal operating cycle, if longer) after its issuance and that does not otherwise qualify as long-term debt. It typically is comprised of borrowings under letters of credit, lines of credit, commercial paper, and notes payable of short duration. Disclosures include amounts of borrowings under each arrangement, description of underlying arrangements, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements such as the effects of refinancings and noncompliance with debt covenants.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	duration

- Details

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- Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: