Debt - Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jul. 11, 2024	Jun. 20, 2024	Jun. 19, 2024	Apr. 01, 2024	Dec. 31, 2023	Sep. 30, 2023
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,452,297					$ 2,298,223	$ 2,341,147
Revolving Credit Facility | Cascade Natural Gas
Long-term debt outstanding [Line Items]
Line of credit facility, maximum borrowing capacity			$ 175,000	$ 100,000
Ratio of total debt to total capitalization			65.00%
Revolving Credit Facility | Intermountain Gas Company
Long-term debt outstanding [Line Items]
Line of credit facility, maximum borrowing capacity			$ 175,000	$ 100,000
Ratio of total debt to total capitalization			65.00%
Term Loan Agreement | WBI Energy Transmission
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization					65.00%
Long-term debt					$ 60,000
Interest rate, stated percentage					4.52%
Debt instrument, annual principal payment					$ 4,000
Senior Notes | Montana-Dakota Utilities Co.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization		65.00%
Long-term debt		$ 125,000
Interest rate, stated percentage		5.96%