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Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Public Utilities, Inventory [Line Items]    
Inventories $ 238,979us-gaap_PublicUtilitiesInventory $ 238,007us-gaap_PublicUtilitiesInventory
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 55,491us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember		 53,127us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories 80,963us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember		 86,062us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember
Natural gas    
Public Utilities, Inventory [Line Items]    
Inventories $ 102,525us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryNaturalGasMember		 $ 98,818us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryNaturalGasMember