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		$ 3,510	$ 3,890
Other Postretirement Benefit Plans, Defined Benefit [Member]
Change in Accumulated PostRetirement Benefit Obligation [Line Items]
Defined Benefit Plan, Expected Future Benefit Payments in 2013		78
Projected benefit obligation, beginning of year	1,868	1,868	1,742
Service cost		51	49	47
Interest cost		91	91	95
Plan amendments		(207)	(3)
Actuarial loss		12	60
Medicare Part D subsidy		5	4
Benefits paid		(68)	(75)
Projected benefit obligation, end of year		1,752	1,868	1,742
Defined Benefit Plan, Fair Value of Plan Assets		756	651	682
Actual return on plan assets		88	(17)
Company contributions	120	95	66
Benefits paid		(78)	(80)
Funded status of the plan		(996)	(1,217)
Noncurrent liabilities		(996)	(1,217)
Net actuarial loss		560	633
Prior service cost		(244)	(53)
Net transition obligation		0	2
Regulatory Assets		316	582
Defined Benefit Plan, Expected Future Benefit Payments in 2014		82
Defined Benefit Plan, Expected Future Benefit Payments in 2015		87
Defined Benefit Plan, Expected Future Benefit Payments in 2016		91
Defined Benefit Plan, Expected Future Benefit Payments in 2017		97
Defined Benefit Plan, Expected Future Benefit Payments in 2018-2022		$ 556