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North Dakota Public Service Commission (NDPSC) (Details 2) - NDPSC - USD ($) $ in Millions | Sep. 26, 2018 | Aug. 10, 2018 | Mar. 09, 2018 | Mar. 01, 2018 | Feb. 14, 2018 | Sep. 06, 2017 | Jul. 21, 2017
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Natural gas rate proceeding |  |  |  |  |  |  |
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% |
Natural gas rate proceeding | Tax Cuts and Jobs Act |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Public utilities interm rate increase (decrese) revised |  |  |  |  | $ 2.7 |  |
Public utilities, requested rate increase (decrease), amended, amount |  |  |  | $ 3.6 |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | $ 2.5 |  |  |  |  |  |
Public utilities, approved rate increase (decrease), percentage | 2.30% |  |  |  |  |  |
Electric rate proceeding | Tax Cuts and Jobs Act |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  | $ (7.2) |  |  |  |
Public utilities, requested rate increase (decrease), percentage |  |  | (3.90%) |  |  |  |
Public utilities, requested rate increase (decrease), amended, amount |  | $ (8.4) |  |  |  |  |

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