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Financial Risk Management Activities (Narrative) (Detail) - Power [Member] - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Fair Value of Derivatives with credit-risk related contingent features $ 20 $ 30
Aggregate fair value of derivative contracts in a liability position that contains triggers for additional collateral 9 13
Additional collateral aggregate fair value $ 11 $ 17