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Organization, Summary of Significant Accounting Policies and Recent Developments (Schedule of Depreciation Rates) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Source of Supply [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	1.15%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_SourceOfSupplyMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.15%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_SourceOfSupplyMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.15%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_SourceOfSupplyMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Source of Supply [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	3.44%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_SourceOfSupplyMember / us-gaap_RangeAxis = us-gaap_MaximumMember	3.44%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_SourceOfSupplyMember / us-gaap_RangeAxis = us-gaap_MaximumMember	3.44%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_SourceOfSupplyMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Pumping [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.87%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_PumpingMember / us-gaap_RangeAxis = us-gaap_MinimumMember	2.87%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_PumpingMember / us-gaap_RangeAxis = us-gaap_MinimumMember	2.87%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_PumpingMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Pumping [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	5.39%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_PumpingMember / us-gaap_RangeAxis = us-gaap_MaximumMember	5.39%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_PumpingMember / us-gaap_RangeAxis = us-gaap_MaximumMember	5.39%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_PumpingMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Water Treatment [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	1.65%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_WaterTreatmentMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.65%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_WaterTreatmentMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.65%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_WaterTreatmentMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Water Treatment [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	7.09%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_WaterTreatmentMember / us-gaap_RangeAxis = us-gaap_MaximumMember	7.09%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_WaterTreatmentMember / us-gaap_RangeAxis = us-gaap_MaximumMember	7.09%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_WaterTreatmentMember / us-gaap_RangeAxis = us-gaap_MaximumMember
General Plant [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.08%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_GeneralPlantMember / us-gaap_RangeAxis = us-gaap_MinimumMember	2.08%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_GeneralPlantMember / us-gaap_RangeAxis = us-gaap_MinimumMember	2.08%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_GeneralPlantMember / us-gaap_RangeAxis = us-gaap_MinimumMember
General Plant [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	17.84%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_GeneralPlantMember / us-gaap_RangeAxis = us-gaap_MaximumMember	17.84%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_GeneralPlantMember / us-gaap_RangeAxis = us-gaap_MaximumMember	17.84%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_GeneralPlantMember / us-gaap_RangeAxis = us-gaap_MaximumMember
T&D Mains [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	1.10%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDMainsMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.10%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDMainsMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.10%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDMainsMember / us-gaap_RangeAxis = us-gaap_MinimumMember
T&D Mains [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	3.13%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDMainsMember / us-gaap_RangeAxis = us-gaap_MaximumMember	3.13%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDMainsMember / us-gaap_RangeAxis = us-gaap_MaximumMember	3.13%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDMainsMember / us-gaap_RangeAxis = us-gaap_MaximumMember
T&D Services [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.12%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDServicesMember / us-gaap_RangeAxis = us-gaap_MinimumMember	2.12%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDServicesMember / us-gaap_RangeAxis = us-gaap_MinimumMember	2.12%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDServicesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
T&D Services [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	3.16%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDServicesMember / us-gaap_RangeAxis = us-gaap_MaximumMember	3.16%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDServicesMember / us-gaap_RangeAxis = us-gaap_MaximumMember	3.16%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDServicesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
T&D Other [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	1.61%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDOtherMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.61%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDOtherMember / us-gaap_RangeAxis = us-gaap_MinimumMember	1.61%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDOtherMember / us-gaap_RangeAxis = us-gaap_MinimumMember
T&D Other [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	4.63%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDOtherMember / us-gaap_RangeAxis = us-gaap_MaximumMember	4.63%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDOtherMember / us-gaap_RangeAxis = us-gaap_MaximumMember	4.63%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / us-gaap_PublicUtilityAxis = msex_TDOtherMember / us-gaap_RangeAxis = us-gaap_MaximumMember

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- Definition

Composite depreciation rate for public utility plants in service.

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