Budgeted construction cost for construction program, next year.

No definition available.

Name:	msex_BudgetedConstructionCost
Namespace Prefix:	msex_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Estimated incremental borrowing rate.

+ References

No definition available.

+ Details

Name:	msex_EstimatedIncrementalBorrowingRate
Namespace Prefix:	msex_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The expiration date of a long-term purchase commitment.

+ References

No definition available.

+ Details

Name:	msex_LongtermPurchaseCommitmentExpirationDate
Namespace Prefix:	msex_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15)
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121555860&loc=d3e11019-110243

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121555860&loc=d3e11049-110243

+ Details

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

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The minimum amount the entity agreed to spend under the long-term purchase commitment.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of units of goods that must be purchased within the specified time period of the agreement to avoid penalties or unfavorable effects on pricing, among other adverse consequences.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

- Details

Name:	us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis=us-gaap_PurchaseCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis=msex_PurchaseCommitment1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis=msex_PurchaseCommitment2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingent Liabilities (Narrative) (Details) gal in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) gal	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Guaranty liabilty for AWM's performance	$ 1.3		$ 1.3		$ 1.4
Budgeted construction cost for construction program, 2020			119.0
Contractual construction agreements			43.0
Rental expenses under operating leases	$ 0.2	$ 0.1	$ 0.4	$ 0.3
Estimated incremental borrowing rate	4.03%		4.03%
NJ Water Supply Authority [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment expiration date of contract			Nov. 30, 2023
Water purchase per commitment \| gal			27.0
Regulated Water Authority [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment expiration date of contract			Feb. 28, 2026
Water purchase per commitment \| gal			3.0
City of Dover [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Water purchase per commitment \| gal			15.0