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Organization, Summary of Significant Accounting Policies and Recent Developments (Schedule of AFUDC Rates) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Middlesex [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Allowance for Funds Used During Construction Rates	6.73%	6.73%	6.72%
Tidewater [Member]
Organization, Summary Of Significant Accounting Policies And Recent Developments [Line Items]
Allowance for Funds Used During Construction Rates	7.92%	7.92%	7.92%

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- Definition

Organization, Summary of Significant Accounting Policies And Recent Developments [Line Items].

+ References

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Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

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