Financing Activities (Details) $ in Millions	Jun. 30, 2025 USD ($)
Montana First Mortgage Bonds Due 2030
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 400.0
Debt Instrument, Interest Rate, Stated Percentage	5.07%
Long-term Debt, Maturity Date	Mar. 21, 2030
Montana First Mortgage Bonds Due 2025
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 161.0
Debt Instrument, Interest Rate, Stated Percentage	5.01%
Long-term Debt, Maturity Date	May 01, 2025
Montana First Mortgage Bonds Due July 2025
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 75.0
Debt Instrument, Interest Rate, Stated Percentage	3.11%
Long-term Debt, Maturity Date	Jul. 01, 2025
Term Loan Due 2026
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100.0
Long-term Debt, Maturity Date	Apr. 10, 2026
South Dakota First Mortgage Bonds Due 2035
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100.0
Debt Instrument, Interest Rate, Stated Percentage	5.49%
Long-term Debt, Maturity Date	May 01, 2035
South Dakota First Mortgage Bonds Due 2025
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 64.0
Debt Instrument, Interest Rate, Stated Percentage	5.01%
Long-term Debt, Maturity Date	May 01, 2025