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Rate and Regulatory Matters (Schedule of Regulatory Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory Assets	$ 58,423	$ 60,894
Retirement Benefits [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 43,070	$ 40,603
Remaining recovery period	Various	Various
Income Taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 9,876	$ 15,899
Remaining recovery period	Various	Various
Rate Cases, Tank Painting, and Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 5,477	$ 4,392
Remaining recovery period	2-10 years	2-10 years

X

- Definition

The period remaining for recovery of costs in the rates we charge customers.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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X

- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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- Details

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