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Capitalization (Schedule of Carrying Amount and Fair Value of Bonds) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Carrying amount [Member]
First Mortgage Bonds	$ 88,271msex_FirstMortgageBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	$ 88,628msex_FirstMortgageBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
SRF Bonds	535msex_StateRevolvingFundBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	540msex_StateRevolvingFundBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Fair value [Member]
First Mortgage Bonds	90,557msex_FirstMortgageBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	90,115msex_FirstMortgageBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
SRF Bonds	$ 537msex_StateRevolvingFundBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	$ 542msex_StateRevolvingFundBondsFairValueDisclosure / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember

X

- Definition

Represents the aggregate of the first mortgage bonds reported on the balance sheet at period end measured at fair value by the entity.

+ References

No definition available.

+ Details

Name:	msex_FirstMortgageBondsFairValueDisclosure
Namespace Prefix:	msex_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Represents the aggregate of the state revolving fund reported on the balance sheet at period end measured at fair value by the entity.

+ References

No definition available.

+ Details

Name:	msex_StateRevolvingFundBondsFairValueDisclosure
Namespace Prefix:	msex_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: