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NDPSC (Details 4) - NDPSC [Member] - USD ($) $ in Millions | Feb. 14, 2018 | Sep. 06, 2017 | Jul. 21, 2017 | Jun. 30, 2017
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Advance determination of prudence [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Estimated project cost |  |  |  | $ 85.0
Natural gas rate proceeding [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  | $ 5.9 |
Public utilities, requested rate increase (decrease), percentage |  |  | 5.40% |
Public utilities, interim rate increase (decrease), amount |  | $ 4.6 |  |
Public utilities, interim rate increase (decrease), percentage |  | 4.20% |  |
Subsequent event [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Public utilities interim rate increase (decrease), revised | $ 2.7 |  |  |

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