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Significant Accounting Policies Property plant equipment (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Interest costs, capitalized during period $ 10.8us-gaap_InterestCostsCapitalized $ 8.2us-gaap_InterestCostsCapitalized $ 7.9us-gaap_InterestCostsCapitalized
Property, plant and equipment, disclosure of composite depreciation rate for plants in service 2.90%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService 3.20%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService 3.30%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService
MONTANA      
Property, Plant and Equipment [Line Items]      
Allowance for funds used during construction, rate 8.00%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate
/ us-gaap_StatementGeographicalAxis
= stpr_MT		 8.10%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate
/ us-gaap_StatementGeographicalAxis
= stpr_MT		 8.00%us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRate
/ us-gaap_StatementGeographicalAxis
= stpr_MT