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Commitments and Contingent Liabilities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Commitment And Contingencies [Line Items]    
Period of wastewater agreement 20 years  
Term of water and wastewater system agreement USA-PA operates the City of Perth Amboy, NJ's water and wastewater systems under a 20-year agreement, which expires in 2018. In connection with the agreement with Perth Amboy, USA-PA entered into a 20-year subcontract with a wastewater operating company  
Guarant liabilty for AWM's performance $ 0.3us-gaap_GuarantyLiabilities $ 0.4us-gaap_GuarantyLiabilities
Budgeted construction cost for construction program, 2015 28.6msex_BudgetedConstructionCost  
Budgeted construction cost for construction program, 2016 33.9msex_BudgetedConstructionCostForConstructionProgramYearOne  
Budgeted construction cost for construction program, 2017 $ 33.1msex_BudgetedConstructionCostForConstructionProgramYearTwo  
NJ Water Supply Authority [Member]    
Commitment And Contingencies [Line Items]    
Commitment of purchase of untreated water Middlesex has an agreement with the NJWSA for the purchase of untreated water through November 30, 2023, which provides for an average purchase of 27 million gallons a day (mgd). Pricing is set annually by the NJWSA through a public rate making process.  
Purchase commitment expiration date of contract Nov. 30, 2023  
Water purchase per commitment 27,000,000us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired
/ us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis
= us-gaap_PurchaseCommitmentMember		  
Regulated Water Authority [Member]    
Commitment And Contingencies [Line Items]    
Minimum treated water purchase commitment Middlesex also has an agreement with a non-affiliated regulated water utility for the purchase of treated water. This agreement, which expires February 27, 2016, provides for the minimum purchase of 3 mgd of treated water with provisions for additional purchases.  
Purchase commitment expiration date of contract Feb. 27, 2016  
Water purchase per commitment 3,000,000us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired
/ us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis
= msex_PurchaseCommitment1Member		  
City of Dover [Member]    
Commitment And Contingencies [Line Items]    
Water purchase per commitment 15,000,000us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired
/ us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis
= msex_PurchaseCommitment2Member