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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 $ 43,231us-gaap_PublicUtilitiesInventory $ 37,138us-gaap_PublicUtilitiesInventory
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 24,738us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember		 21,600us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories $ 18,493us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember		 $ 15,538us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember