Capitalization (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Carrying amount
First Mortgage Bonds	$ 89,565	$ 91,938
SRF Bonds	701	708
Fair value
First Mortgage Bonds	84,195	93,556
SRF Bonds	$ 704	$ 712

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