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Subsequent event (Details) - USD ($) $ in Thousands | Jul. 11, 2024 | Jun. 30, 2024 | Dec. 31, 2023 | Jun. 30, 2023
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Subsequent Events [Line Items] |  |  |  |
Long-term debt |  | $ 2,400,617 | $ 2,298,223 | $ 2,247,422
Senior Notes | Montana-Dakota Utilities Co. | Subsequent Event |  |  |  |
Subsequent Events [Line Items] |  |  |  |
Long-term debt | $ 125,000 |  |  |
Interest rate, stated percentage | 5.96% |  |  |
Ratio of total debt to total capitalization | 65.00% |  |  |

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