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Commitments and Contingent Liabilities (Schedule of Purchased Water Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Purchased Water
Treated	$ 791	$ 804	$ 2,365	$ 2,360
Untreated	673	675	1,905	1,911
Total Costs	$ 1,464	$ 1,479	$ 4,270	$ 4,271

X

- 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

X

- 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=63488584&loc=d3e20235-122688

+ Details

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