Capitalization (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Carrying amount
First Mortgage Bonds	$ 90,208	$ 86,577
SRF Bonds	785	793
Fair value
First Mortgage Bonds	91,370	87,283
SRF Bonds	$ 790	$ 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