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Supplementary Financial Information (Nuclear Decommissioning Trust) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Schedule of Schedule of Decommissioning Fund Investments [Line Items]          
Cost [1] $ 593   $ 593   $ 564
Unrealized gain 290   290   333
Unrealized loss (9)   (9)   (4)
Fair market value 874   874   893
Realized gains 1 $ 1 2 $ 2  
Realized losses (2) 0 (3) (1)  
Proceeds from sales of securities 242 165 315 250  
Investments in securities (247) $ (170) (328) $ (263)  
Debt securities [Member]          
Schedule of Schedule of Decommissioning Fund Investments [Line Items]          
Cost [1],[2] 304   304   288
Unrealized gain [2] 11   11   13
Unrealized loss [2] (1)   (1)   0
Fair market value [2] $ 314   $ 314   $ 301
Debt securities, average coupon rate (as a percent) 3.58%   3.58%   4.35%
Decommissioning Fund Investments, Debt securities, average maturity     7 years   6 years
Decommissioning Fund Investments, debt maturities, one through five years, fair value $ 96   $ 96    
Decommissioning Fund Investments, debt maturities, five through ten years, fair value 85   85    
Decommissioning Fund Investments, debt maturities, after ten years, fair value 133   133    
Equity securities [Member]          
Schedule of Schedule of Decommissioning Fund Investments [Line Items]          
Cost [1],[3] 289   289   $ 276
Unrealized gain [3] 279   279   320
Unrealized loss [3] (8)   (8)   (4)
Fair market value [3] $ 560   $ 560   $ 592
[1] Includes realized gains and losses on securities sold.
[2] The investment objective for debt securities is to invest in a diversified tax efficient portfolio with an overall portfolio rating of AA or above as graded by S&P or Aa2 by Moody's Investors Services, Inc. The debt securities are heavily weighted with municipal bonds. The debt securities had an average coupon rate of 3.58% and 4.35% at September 30, 2015 and December 31, 2014, respectively, and an average maturity of 7 years and 6 years at September 30, 2015 and December 31, 2014, respectively.
[3] The investment objective for equity securities is to invest tax efficiently and to match the performance of the S&P 500 Index.