Trust Investments (Proceeds From The Sales Of And The Net Realized Gains On Securities in the NDT and Rabbi Trusts) (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Schedule of Trust Investments [Line Items]    
Net Gains (Losses) on Trust Investments $ (22) $ 28
Rabbi Trust [Member]    
Schedule of Trust Investments [Line Items]    
Proceeds from Sales [1] 25 51
Gross Realized Gains 2 15
Gross Realized Losses (2) (3)
Net Realized Gains (Losses) [2] 0 12
Unrealized Gain (Loss) on Securities [3] 0  
Other than Temporary Impairment Losses, Investments 0 0
Net Gains (Losses) on Trust Investments 0 12
Power [Member]    
Schedule of Trust Investments [Line Items]    
Net Gains (Losses) on Trust Investments (22) 19
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]    
Schedule of Trust Investments [Line Items]    
Proceeds from Sales [4] 372 247
Gross Realized Gains 24 21
Gross Realized Losses (12) (4)
Net Realized Gains (Losses) [5] 12 17
Unrealized Gain (Loss) on Securities [6] (34)  
Other than Temporary Impairment Losses, Investments 0 (1)
Net Gains (Losses) on Trust Investments $ (22) $ 16
[1] Includes activity in accounts related to the liquidation of funds being transitioned to new managers.
[2] The cost of these securities was determined on the basis of specific identification.
[3] Effective January 1, 2018, unrealized gains (losses) on equity securities are recorded in Net Income instead of Other Comprehensive Income (Loss).
[4] Includes activity in accounts related to the liquidation of funds being transitioned to new managers.
[5] The cost of these securities was determined on the basis of specific identification.
[6] Effective January 1, 2018, unrealized gains (losses) on equity securities are recorded in Net Income instead of Other Comprehensive Income (Loss).