Capitalization (Schedule of Carrying Amount and Fair Value of Bonds) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount [Member]
First Mortgage Bonds	$ 104,449	$ 95,322
Fair Value [Member]
First Mortgage Bonds	$ 106,188	$ 98,036

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

No definition available.

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