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Summary of Significant Accounting Policies - NPC - Inventory (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Inventory [Line Items]    
Inventory, Raw Materials and Supplies, Gross $ 523 $ 529
Energy Related Inventory, Other Fossil Fuel 402 353
Nevada Power Company [Member]    
Inventory [Line Items]    
Inventory, Raw Materials and Supplies, Gross 60  
Energy Related Inventory, Other Fossil Fuel $ 13 $ 22