Significant Accounting Policies Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) leases Months	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Number of capital leases reclassified to financing | leases	1
Property, plant and equipment, disclosure of composite depreciation rate for plant in service	2.80%	3.00%	3.00%
Allowance for doubtful accounts receivable, current	$ 2.3	$ 2.3
Unbilled receivables,current	$ 83.3	78.2
Number of months or less of maturity to be considered cash equivalent | Months	3
Interest Costs Capitalized	$ 8.2	$ 5.9	$ 8.5
South Dakota
Allowance for funds used during construction, rate	6.60%	6.70%	7.20%
Montana
Allowance for funds used during construction, rate	6.90%	7.10%	7.20%