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Selected Balance Sheet Data, Inventory (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Public Utilities, Inventory [Line Items]    
Inventories $ 14,855us-gaap_PublicUtilitiesInventory $ 24,685us-gaap_PublicUtilitiesInventory
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 6,661us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember		 6,494us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventorySuppliesMember
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories 6,958us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember		 6,654us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryFuelMember
Natural gas    
Public Utilities, Inventory [Line Items]    
Inventories $ 1,236us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryNaturalGasMember		 $ 11,537us-gaap_PublicUtilitiesInventory
/ us-gaap_PublicUtilitiesInventoryAxis
= us-gaap_PublicUtilitiesInventoryNaturalGasMember