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Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 5,729,671us-gaap_PropertyPlantAndEquipmentGross $ 5,615,125us-gaap_PropertyPlantAndEquipmentGross
Less accumulated depreciation (1,898,627)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment (1,871,984)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Property, plant and equipment, net 3,831,044us-gaap_PropertyPlantAndEquipmentNet 3,743,141us-gaap_PropertyPlantAndEquipmentNet
Electric plant    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 5,480,750us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember		 5,376,606us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
Construction work in progress    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 248,921us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember		 $ 238,519us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember