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North Dakota Public Service Commission (NDPSC) (Details 4) - NDPSC [Member] - USD ($) $ in Millions | Sep. 06, 2017 | Jul. 26, 2017 | Jul. 21, 2017 | Jun. 30, 2017 | Jun. 16, 2017 | Nov. 21, 2016 | Oct. 14, 2016 | Mar. 09, 2016 | Feb. 10, 2016 | Jan. 05, 2016
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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% |  |  |  |  |  |  |  |  |
Electric rate proceeding [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  |  |  |  | $ 13.4 |  |  |
Public utilities, requested rate increase (decrease), percentage |  |  |  |  |  |  | 6.60% |  |  |
Public utilities, interim rate increase (decrease), amount |  |  |  |  |  | $ 11.7 | $ 13.0 |  |  |
Public utilities, interim rate increase (decrease), percentage |  |  |  |  |  | 5.80% | 6.50% |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  |  | $ 7.5 |  |  |  |  |
Public utilities, approved rate increase (decrease), percentage |  |  |  |  | 3.70% |  |  |  |  |
Public utilities refund from current rates, percentage |  | 19.00% |  |  |  |  |  |  |  |
Electric rider [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  | 9.45% |  |  |  |  |
Electric renewable cost rider [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Public utilities, interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  | $ 15.1
Public utilities, approved return on equity, percentage |  |  |  |  | 9.65% |  |  |  |  |
Public utilities, requested return on equity, percentage |  |  |  |  |  |  |  |  |  | 10.50%
Electric transmission adjustment [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Public utilities, interim rate increase (decrease), amount |  |  |  |  |  |  |  |  | $ 6.8 |
Public utilities, requested rate increase (decrease), amended, percentage |  |  |  |  |  |  |  |  | 10.50% |
Electric generation rider [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Public utilities, interim rate increase (decrease), amount |  |  |  |  |  |  |  | $ 9.7 |  |
Public utilities, approved return on equity, percentage |  |  |  |  | 9.45% |  |  |  |  |
Public utilities, requested return on equity, percentage |  |  |  |  |  |  |  | 10.50% |  |

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