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Capitalization (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying amount
First Mortgage Bonds	$ 85,590	$ 91,938
SRF Bonds	701	708
Fair value
First Mortgage Bonds	87,714	93,556
SRF Bonds	$ 705	$ 712

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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