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Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail)
$ in Millions
Dec. 31, 2016
USD ($)
Rabbi Trust [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale debt securities, Less than one year $ 8
Available-for-sale debt securities, 1-5 years 44
Available-for-sale debt securities, 6-10 years 44
Available-for-sale debt securities, 11-15 years 9
Available-for-sale debt securities, 16-20 years 8
Available-for-sale debt securities, Over 20 years 81
Total Available-for-Sale Debt Securities 194
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Available-for-sale debt securities, Less than one year 15
Available-for-sale debt securities, 1-5 years 257
Available-for-sale debt securities, 6-10 years 193
Available-for-sale debt securities, 11-15 years 50
Available-for-sale debt securities, 16-20 years 60
Available-for-sale debt securities, Over 20 years 282
Total Available-for-Sale Debt Securities $ 857