Significant Accounting Policies Property plant equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property, plant and equipment, disclosure of composite depreciation rate for plant in service	2.90%	2.80%	2.80%
Allowance for Funds Used During Construction, Capitalized Interest	$ 27.1	$ 24.3	$ 20.2
Montana
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	7.00%	6.40%	6.40%
South Dakota
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	6.90%	6.40%	6.40%
Minimum [Member] | Property, Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Maximum [Member] | Property, Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	127 years