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NDPSC (Details 3) - Gas Distribution [Member] - NDPSC [Member] - Montana-Dakota Utilities Co. [Member] - USD ($) | Jun. 01, 2021 | Mar. 31, 2021 | Mar. 02, 2021 | Jan. 01, 2021 | Aug. 26, 2020
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Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  | $ 9,000,000.0
Public Utilities, Requested Rate Increase (Decrease), Percentage |  |  |  |  | 7.80%
Public Utilities, Interim Rate Increase (Decrease), Amount |  |  |  | $ 6,900,000 |
Public Utilities, Interim Rate Increase (Decrease), Percentage |  |  |  | 6.00% |
Generation Resource Recovery Rider |  |  | $ 0 |  |
Subsequent Event [Member] |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | $ 6,900,000 |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Percentage | 6.00% |  |  |  |
Pending Rate Case [Member] |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  | $ 13,500,000 |  |  |

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