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Summary of Significant Accounting Policies (Regulatory Rate Filings) (Details) - USD ($) |  |  |  |  |  |  |  |  |  | 1 Months Ended | 11 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 06, 2018 | Nov. 29, 2017 | Nov. 15, 2017 | Jun. 26, 2017 | Jun. 01, 2017 | Apr. 03, 2017 | Feb. 17, 2017 | Nov. 15, 2016 | Dec. 03, 2015 | Jun. 30, 2016 | Nov. 28, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 04, 2018 | Sep. 27, 2017 | Jun. 06, 2017
Public Utilities General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing Account, Revenue Increase (Reduction) |  |  |  |  |  |  |  |  |  |  |  | $ 3,137,000 | $ 11,024,000 | $ 2,745,000 |  |  |
Authorized Regulatory Surcharge, Per 15 CCF |  |  |  |  |  |  | $ 1.52 |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  | $ 7,550,000 | $ 5,339,000 | $ 13,205,000 |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Percentage |  |  |  |  |  | 2.10% | 1.50% | 3.80% |  |  |  |  |  |  |  |  |
Water Reduction Target Goal, Percentage |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Percentage |  |  | 4.22% | 3.46% |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Service Charge Refund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,794,000
Public Utilities, Approved Reimbursement Fee, Percentage |  | 1.40% |  |  |  |  |  |  |  |  | 1.44% |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  | $ 15,670,000 | $ 12,407,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing and Memorandum Account, Surcharge Offset |  |  |  |  |  |  |  |  |  |  |  | 1,035,000 | 0 | 0 |  |  |
Regulatory Balancing and Memorandum Accounts, Net Under-Collected |  |  |  |  |  |  |  |  |  |  |  | $ 3,738,000 |  |  |  | $ 11,474,000 |
Regulatory Balancing and Memorandum Accounts Review, Authorized Revenue, Threshold Percentage |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  |  |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  | $ 99,554,000 | 151,773,000 |  |  |  |
Current regulatory assets, net |  |  |  |  |  |  |  |  |  |  |  | 0 | 16,064,000 |  |  |  |
Total regulatory assets, net, less current portion |  |  |  |  |  |  |  |  |  |  |  | 99,554,000 | 135,709,000 |  |  |  |
Public Utilities, Requested Return on Equity, Percentage |  |  |  |  |  | 9.43% |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Equity Capital Structure, Percentage |  |  |  |  |  | 8.09% |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  |  | 62,476,000 | 0 |  |  |  |
Deferred Income Tax Charge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  |  |  |  |  |  | 0 | 10,139,000 |  |  |  |
Postretirement pensions and other medical benefits [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  |  |  |  |  |  | 68,556,000 | 109,795,000 |  |  |  |
Balancing and Memorandum Accounts [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  |  |  |  |  |  | 27,925,000 | 25,909,000 |  |  |  |
Other Regulatory Assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing Account, Revenue Increase (Reduction) |  |  |  |  |  |  |  |  |  |  |  | (678,000) | (483,000) | (293,000) |  |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  |  |  |  |  |  | 3,073,000 | 5,930,000 |  |  |  |
2014 WCMA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Memorandum Account, Authorized Recovery |  |  |  |  | $ 996,000 |  |  |  | $ 4,259,000 |  |  |  |  |  |  |  |
Authorized Regulatory Surcharge, Per CCF |  |  |  |  | 0.08 |  |  |  | $ 0.08 |  |  |  |  |  |  |  |
WCMA Reserve |  |  |  |  |  |  |  |  |  |  |  | 0 | 1,090,000 | 1,278,000 |  |  |
2015 WCMA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Memorandum Account, Authorized Recovery |  |  |  |  | 2,233,000 |  |  |  |  |  |  |  |  | 19,854,000 |  |  |
Authorized Regulatory Surcharge, Per CCF |  |  |  |  | $ 0.1441 |  |  |  |  |  |  |  |  |  |  |  |
WCMA Reserve |  |  |  |  |  |  |  |  |  |  |  | 0 | 2,115,000 | 2,343,000 |  |  |
Balancing and Memorandum Account, Surcharge Offset |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,139,000 |  |  |
Authorized Regulatory Surcharge, Net |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,372,000 |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase for 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 34,288,000 |  |
Requested rate increase for 2016 as percentage of total revenue at time of request |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.76% |  |
Requested rate increase for 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,232,000 |  |
Requested rate increase for 2017 as percentage of total revenue at time of request |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.70% |  |
Requested rate increase for 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,582,000 |  |
Requested rate increase for 2018 as percentage of total revenue at time of request |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.17% |  |
Memorandum Account, Authorized Recovery |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,725,000 |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Proposed Return on Equity, Percentage | 8.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Proposed Equity Capital Structure, Percentage | 7.19% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount | $ (10,000,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred Income Tax Charge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets and Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  |  | $ 62,476,000 | $ 0 |  |  |  |

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