Retirement Benefits and Trusteed Assets (Details 2) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Regulatory Assets		$ 3,510	$ 3,890
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year		3,307	2,963
Projected benefit obligation, beginning of year	3,196	3,196	2,899
Service cost		64	55	52
Interest cost		155	154	153
Actuarial loss		342	251
Settlements		2	2	0
Benefits paid		(174)	(165)
Projected benefit obligation, end of year		3,585	3,196	2,899
Plan assets at fair value, beginning of the yeard Benefit Plan, Fair Value of Plan Assets	1,957	1,957	1,936
Actual return on plan assets		220	(18)
Company contributions	275	208	204
Plan assets at fair value, end of the yeard Benefit Plan, Fair Value of Plan Assets		2,211	1,957	1,936
Funded status of the plan		(1,374)		(1,239)
Current liabilities		(6)	(8)
Noncurrent liabilities		(1,368)	(1,231)
Pension and Other Postretirement Defined Benefit Plans, Liabilities		(1,374)	(1,239)
Net actuarial loss		1,805	1,645
Prior service cost		10	11
Regulatory Assets		1,815	1,656
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Projected benefit obligation, beginning of year	1,868	1,868	1,742
Service cost		51	49	47
Interest cost		91	91	95
Actuarial loss		12	60
Benefits paid		(68)	(75)
Projected benefit obligation, end of year		1,752	1,868	1,742
Plan assets at fair value, beginning of the yeard Benefit Plan, Fair Value of Plan Assets	651	651	682
Actual return on plan assets		88	(17)
Company contributions	120	95	66
Plan assets at fair value, end of the yeard Benefit Plan, Fair Value of Plan Assets		756	651	682
Funded status of the plan		(996)	(1,217)
Noncurrent liabilities		(996)	(1,217)
Net actuarial loss		560	633
Prior service cost		(244)	(53)
Regulatory Assets		$ 316	$ 582