Significant Accounting Policies Property plant equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Interest Costs Capitalized	$ 8.2	$ 5.9	$ 8.5
Property, plant and equipment, disclosure of composite depreciation rate for plant in service	2.80%	3.00%	3.00%
Montana
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	6.90%	7.10%	7.20%
South Dakota
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	6.60%	6.70%	7.20%
Minimum [Member] | Property, Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life		3 years
Maximum [Member] | Property, Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	96 years