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NDPSC (Details 3) - NDPSC [Member] - USD ($) | Jan. 23, 2019 | Jan. 09, 2019 | Nov. 20, 2018 | Sep. 26, 2018 | Mar. 09, 2018 | Mar. 01, 2018 | Feb. 14, 2018 | Sep. 06, 2017 | Jul. 21, 2017
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Natural gas rate proceeding [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  |  |  |  |  |  | $ 5,900,000
Public utilities, requested rate increase (decrease), percentage |  |  |  |  |  |  |  |  | 5.40%
Public utilities, interim rate increase (decrease), amount |  |  |  |  |  |  |  | $ 4,600,000 |
Public utilities, interim rate increase (decrease), percentage |  |  |  |  |  |  |  | 4.20% |
Tax Cuts and Jobs Act [Member] | Natural gas rate proceeding [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Public utilities interim rate increase (decrease), revised |  |  |  |  |  |  | $ 2,700,000 |  |
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount |  |  |  |  |  | $ 3,600,000 |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  | $ 2,500,000 |  |  |  |  |
Public utilities, approved rate increase (decrease), percentage |  |  |  | 2.30% |  |  |  |  |
Tax Cuts and Jobs Act [Member] | Electric Rate Proceeding [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  |  | $ (7,200,000) |  |  |  |
Public utilities, requested rate increase (decrease), percentage |  |  |  |  | (3.90%) |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  | $ (8,400,000) |  |  |  |  |
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount |  |  | $ (7,900,000) |  |  |  |  |  |
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Percentage |  |  | (4.70%) |  |  |  |  |  |
Tax Cuts and Jobs Act [Member] | Subsequent event [Member] | Natural gas rate proceeding [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  | $ (168,000) |  |  |  |  |  |  |
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount | $ (200,000) |  |  |  |  |  |  |  |

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