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Organization, Summary of Significant Accounting Policies and Recent Developments (Schedule of Depreciation Rates) (Details) | 12 Months Ended
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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. + References No definition available. + Details
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