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Commitments and Contingent Liabilities (Schedule of Purchased Water Costs) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Purchased Water |  |  |
Untreated | $ 3,600 | $ 2,800 | $ 2,600
Treated | 3,200 | 3,300 | 3,200
Total Costs | $ 6,800 | $ 6,100 | $ 5,800

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- Definition Purchased Water + References No definition available. + 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

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- 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

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- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 + Details
Name: | us-gaap_CostOfPurchasedWater
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration