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Long-term borrowings (Details 4) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 16, 2020 | Oct. 30, 2020 | Jun. 15, 2020 | Dec. 31, 2019 | Jul. 26, 2019
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Long-term debt outstanding [Line Items] |  |  |  |  |  |
Long-term debt | $ 2,213,130 |  |  |  | $ 2,243,107 |
Montana-Dakota Utilities Co. [Member] |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization | 49.00% |  |  |  |  |
Cascade Natural Gas [Member] |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization | 53.00% |  |  |  |  |
Intermountain Gas Company |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization | 51.00% |  |  |  |  |
Centennial Energy Holdings, Inc. |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization | 31.00% |  |  |  |  |
WBI Energy Transmission, Inc. |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization | 41.00% |  |  |  |  |
Senior Notes |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Long-term debt | $ 1,950,000 |  |  |  | $ 1,850,000 |
Weighted average interest rate | 4.40% |  |  |  |  |
Senior Notes | Cascade Natural Gas [Member] |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Long-term debt |  |  | $ 25,000 | $ 50,000 |  |
Weighted average interest rate |  |  | 3.34% | 3.66% |  |
Ratio of total debt to total capitalization as specified in debt covenants |  |  | 65.00% | 65.00% |  |
Senior Notes | WBI Energy Transmission, Inc. |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Long-term debt | $ 195,000 | $ 25,000 |  |  |  |
Weighted average interest rate |  | 3.26% |  |  |  |
Ratio of total debt to total capitalization as specified in debt covenants | 55.00% |  |  |  |  |
Long-term private shelf agreement issuance capacity |  |  |  |  |  | $ 300,000
Remaining capacity under uncommitted private shelf agreement | $ 105,000 |  |  |  |  |
Revolving credit facility | Montana-Dakota Utilities Co. [Member] |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of funded debt to total capitalization as specified in debt covenants | 65.00% |  |  |  |  |
Revolving credit facility | Cascade Natural Gas [Member] |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt covenants | 65.00% |  |  |  |  |
Revolving credit facility | Intermountain Gas Company |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt covenants | 65.00% |  |  |  |  |
Revolving credit facility | Centennial Energy Holdings, Inc. |  |  |  |  |  |
Long-term debt outstanding [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt covenants | 65.00% |  |  |  |  |

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