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Fair Value Measurements - NPC - Debt (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt $ 39,353 $ 36,774
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value 46,004 39,398
Nevada Power Company [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt 2,351 2,353
Nevada Power Company [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value $ 2,848 $ 2,651