Capitalization (Schedule of Carrying Amount and Fair Value of Bonds) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying amount [Member]
Bonds	$ 147,667	$ 151,361
Fair value [Member]
Bonds	$ 159,195	$ 160,772

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