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Selected Balance Sheet Data, Inventory (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Public Utilities, Inventory [Line Items]    
Inventories $ 24,685us-gaap_PublicUtilitiesInventory $ 21,475us-gaap_PublicUtilitiesInventory
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 6,494us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember		 6,437us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories 6,654us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember		 5,915us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember
Natural gas    
Public Utilities, Inventory [Line Items]    
Inventories $ 11,537us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryNaturalGasMember		 $ 9,123us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryNaturalGasMember