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Commitments and Contingent Liabilities (Schedule of Purchased Water Costs) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Purchased Water |  |  |
Treated | $ 2.4 msex_CostOfTreatedWater | $ 2.4 msex_CostOfTreatedWater | $ 2.4 msex_CostOfTreatedWater
Untreated | 3.1 msex_CostOfUntreatedWater | 3.2 msex_CostOfUntreatedWater | 3.1 msex_CostOfUntreatedWater
Total Costs | $ 5.5 us-gaap_CostOfPurchasedWater | $ 5.6 us-gaap_CostOfPurchasedWater | $ 5.5 us-gaap_CostOfPurchasedWater

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- Details
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Namespace Prefix: | msex_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Refers to cost of treated water during the period. + References No definition available. + Details
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Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Refers to cost of untreated water during the period. + References No definition available. + Details
Name: | msex_CostOfUntreatedWater
Namespace Prefix: | msex_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Cost of water purchased, may include any water related assessment. If water is bought on a unit volume basis, would also be considered as commodity costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_CostOfPurchasedWater
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration