Supplementary Financial Information (Nuclear Decommissioning Trust) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Schedule of Schedule of Decommissioning Fund Investments [Line Items]
Cost	$ 513	[1]		$ 513	[1]		$ 491	[1]
Unrealized gain	236			236			177
Unrealized loss	(12)			(12)			(14)
Fair market value	737			737			654
Realized gains	1		0	2		1
Realized losses	(3)		(1)	(3)		(2)
Proceeds from sales of securities	23		25	128		56
Investments in securities	(28)		(30)	(140)		(68)
Debt securities [Member]
Schedule of Schedule of Decommissioning Fund Investments [Line Items]
Cost	261	[1],[2]		261	[1],[2]		246	[1],[2]
Unrealized gain	9	[2]		9	[2]		16	[2]
Unrealized loss	(4)	[2]		(4)	[2]		(1)	[2]
Fair market value	266	[2]		266	[2]		261	[2]
Debt securities, average coupon rate (as a percent)	3.97%			3.97%			4.38%
Decommissioning Fund Investments, debt maturities, one through five years, fair value	103			103
Decommissioning Fund Investments, debt maturities, five through ten years, fair value	58			58
Decommissioning Fund Investments, debt maturities, after ten years, fair value	105			105
Equity securities [Member]
Schedule of Schedule of Decommissioning Fund Investments [Line Items]
Cost	252	[1],[3]		252	[1],[3]		245	[1],[3]
Unrealized gain	227	[3]		227	[3]		161	[3]
Unrealized loss	(8)	[3]		(8)	[3]		(13)	[3]
Fair market value	$ 471	[3]		$ 471	[3]		$ 393	[3]

[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 3.97% and 4.38% at September 30, 2013 and December 31, 2012, respectively, and an average maturity of 6 years at both September 30, 2013 and December 31, 2012.

[3] The investment objective for equity securities is to invest tax efficiently and to match the performance of the S&P 500 Index.