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Capitalization (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Carrying amount
Bonds	$ 88,275	$ 86,577
SRF Bonds	709	793
Fair value
Bonds	89,422	87,283
SRF Bonds	$ 712	$ 799

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