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Summary of Significant Accounting Policies (Advances for Construction and Contributions in Aid of Construction) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Accounting Policies [Abstract] |  |
Refund period for advances for construction received after 1981 | 40 years |
Estimated Refunds of Advances for Construction [Abstract] |  |
2016 | $ 2,766 |
2017 | 2,766 |
2018 | 2,766 |
2019 | 2,766 |
2020 | 2,766 |
Thereafter | 52,338 |
Customer Advances for Construction | 83,695 | $ 84,815
Customer Advances for Construction, Non-Refundable | $ 17,527 |
Amortization period for taxes paid relating to advances and contributions | 40 years |

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- Definition For utilities only, represents the carrying amount of the liability as of the balance sheet date for payments received by a utility from its customers in advance of performing its obligations under terms of its construction agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 910 -SubTopic 405 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=68049106&loc=d3e50628-109371 + Details
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