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Summary of Significant Accounting Policies - NPC - Inventory (Details)
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
OperatingSegments
TheNumberOfReportableSegments
Dec. 31, 2018
USD ($)
Inventory [Line Items]    
Number of Reportable Segments | OperatingSegments 8  
Inventory, Raw Materials and Supplies, Gross $ 616 $ 571
Energy Related Inventory, Other Fossil Fuel $ 257 273
Nevada Power Company [Member]    
Inventory [Line Items]    
Number of Reportable Segments | TheNumberOfReportableSegments 1  
Inventory, Raw Materials and Supplies, Gross $ 62 56
Energy Related Inventory, Other Fossil Fuel $ 0 $ 5