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Summary of Significant Accounting Policies - SPPC - Inventory (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Inventory [Line Items]    
Inventory, Raw Materials and Supplies, Gross $ 571 $ 536
Energy Related Inventory, Other Fossil Fuel 273 352
Sierra Pacific Power Company [Member]    
Inventory [Line Items]    
Inventory, Raw Materials and Supplies, Gross 44 42
Energy Related Inventory, Other Fossil Fuel $ 8 $ 7