Significant Accounting Policies Property plant equipment (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Interest costs, capitalized during period	$ 7.9	$ 3.1	$ 11.0
Capitalized costs relating to preliminary survey and investigation	1.2	21.8
Contributions in aid of construction	$ 5.0	$ 2.0
Property, plant and equipment, disclosure of composite depreciation rate for plants in service	3.30%	3.30%	3.20%
Montana
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	8.00%	7.90%	8.20%
South Dakota
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	8.00%	7.80%	8.20%