Capitalization (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Carrying amount
Bonds	$ 91,938	$ 86,577
SRF Bonds	708	793
Fair value
Bonds	93,556	87,283
SRF Bonds	$ 712	$ 799

Represents the aggregate of the state revolving fund reported on the balance sheet at period end measured at fair value by the entity.

No definition available.

Name:	MSEX_StateRevolvingFundBondsFairValueDisclosure
Namespace Prefix:	MSEX_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant