Retirement Benefits and Trusteed Assets (Details 10) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Change in Accumulated PostRetirement Benefit Obligation [Line Items]
Regulatory Assets		$ 4,417	$ 4,853
Other Postretirement Benefit Plans, Defined Benefit [Member]
Change in Accumulated PostRetirement Benefit Obligation [Line Items]
Benefit obligation, beginning of year	2,470	2,470	2,305
Service cost		68	64	61
Interest cost		120	121	125
Plan amendments		(264)	(4)
Actuarial loss		5	80
Medicare Part D subsidy		6	6
Benefits paid		(90)	(102)
Benefit obligation, end of year		2,315	2,470	2,305
Defined Benefit Plan, Fair Value of Plan Assets		1,153	985	1,029
Actual return on plan assets		131	(22)
Company contributions	145	140	111
Benefits paid		(103)	(133)
Funded status of the plans		(1,162)	(1,485)
Net actuarial loss		40	47
Prior serice cost (credit)		(14)	(20)
Net transition obligation		(1)	(1)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax		25	26
Net actuarial loss		727	835
Prior service cost		(302)	(60)
Net transition obligation		1	3
Regulatory Assets		426	778
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities		(2)	(1)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent		(1,160)	(1,484)
Pension and Other Postretirement Defined Benefit Plans, Liabilities		$ (1,162)	$ (1,485)