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Rates and Other Regulatory Activities - Additional Information (Detail) | 3 Months Ended |
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Mar. 31, 2018 USD ($) dekatherm / d unit mi | Dec. 31, 2017 USD ($)
Electric Distribution [Member] |  |
Rates and Other Regulatory Activities [Line Items] |  |
Amount of Regulatory Costs Not yet Approved | $ 15,200,000 |
Electric Limited Proceedings [Member] |  |
Rates and Other Regulatory Activities [Line Items] |  |
Asset Recovery Damaged Property Costs, Noncurrent | 1,500,000 |
Deferred Storm and Property Reserve Deficiency, Noncurrent | 800,000 |
Public Utilities, Requested Rate Increase (Decrease), Amount | 1.82 |
Eastern Shore [Member] |  |
Rates and Other Regulatory Activities [Line Items] |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | 9,800,000 |
Cost of Services | 60,000,000 |
Public Utilities, Requested Rate Increase (Decrease), Amount | $ 18,900,000 |
Public Utilities, Requested Return on Equity, Percentage | 13.75% |
Number of Months Rates Suspended | 5 months |
Increase in Revenue Recognized Due to Motion Rate in Effect |  | $ 3,700,000
2017 Expansion Project [Member] | Eastern Shore [Member] |  |
Rates and Other Regulatory Activities [Line Items] |  |
Estimated Capital Cost | $ 117,000,000 |
Number of pipeline miles requested | mi | 23 |
Revised Miles Of Natural Gas Pipeline | mi | 17 |
Pressure Control Stations | 2 |
Firm natural gas transportation deliverability | dekatherm / d | 61,162 |
Additional Firm Natural Gas Transportation Deliverability | dekatherm / d | 52,500 |
Subscribers [Member] | 2017 Expansion Project [Member] | Eastern Shore [Member] |  |
Rates and Other Regulatory Activities [Line Items] |  |
Number of customers | 7 |
Number of affiliates | unit | 3 |

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- Definition Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378 + Details
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