Commitments and Contingent Liabilities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Commitment And Contingencies [Line Items]
Period of wastewater agreement	20 years
Water and wastewater system agreement	USA-PA operates the City of Perth Amboy, New Jersey’s (Perth Amboy) 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 for the operation and maintenance of the Perth Amboy wastewater collection system.
Guarant liabilty for AWM's performance	$ 0.3	$ 0.3
Budgeted construction cost for construction program, 2015	$ 23.6
NJ Water Supply Authority [Member]
Commitment And Contingencies [Line Items]
Commitment to purchase water	Middlesex has an agreement with the New Jersey Water Supply Authority (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	27,000,000
Regulated Water Authority [Member]
Commitment And Contingencies [Line Items]
Commitment to purchase water	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. 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. At the end of the agreement period, it is expected that the agreement will be extended for five years.
Purchase commitment expiration date of contract	Feb. 27, 2016
Water purchase per commitment	3,000,000
City of Dover [Member]
Commitment And Contingencies [Line Items]
Water purchase per commitment	15,000,000