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Risk Management and Hedging Activities Risk Management and Hedging Activities - NPC - Collateral and Contingent Features (Details) - Commodity Contract [Member] - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Derivative [Line Items]    
Derivative, Net Liability Position, Aggregate Fair Value $ (145) $ (190)
Nevada Power Company [Member]    
Derivative [Line Items]    
Derivative, Net Liability Position, Aggregate Fair Value $ 1 $ 2