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Rate and Regulatory Matters (Schedule of Regulatory Assets) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015
Regulatory Assets [Line Items] |  |
Regulatory Assets | $ 60,894 | $ 58,552
Retirement Benefits [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | $ 40,603 | $ 39,378
Remaining recovery period | Various | Various
Income Taxes [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | $ 15,899 | $ 15,541
Remaining recovery period | Various | Various
Rate Cases, Tank Painting, and Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | $ 4,392 | $ 3,633
Remaining recovery period | 2-10 years | 2-10 years

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- 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 + Details
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