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CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (Parenthetical) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY [Abstract] |  |
Conversion of Convertible Preferred Stock Value Per Share | $ 7.00 msex_ConversionOfConvertiblePreferredStockValuePerShare | $ 8.00 msex_ConversionOfConvertiblePreferredStockValuePerShare

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- Definition Refers to per share value of conversion of convertible preferred stock. + References No definition available. + Details
Name: | msex_ConversionOfConvertiblePreferredStockValuePerShare
Namespace Prefix: | msex_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementOfStockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration