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Employee Benefit Plans (Schedule of Amounts Expected to be Amortized from Regulatory Assets to Net Periodic Benefit Cost) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Pension Benefit Plan [Member]
Actuarial Loss	$ 1,618
Other Benefits Plan [Member]
Actuarial Loss	$ 1,319

X

- Definition

Expected amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions.

+ References

No definition available.

+ Details

Name:	msex_ExpectedAmortizedActuarialLoss
Namespace Prefix:	msex_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: