Benefit Plans - Costs of Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Costs Of Benefit Plans [Line Items]
Percentage of actuarial gains and losses in excess	10.00%
Other pension information
Income tax expense due to patient protection and affordable care act	$ 0	$ 22
Pension Benefits
Defined benefit plan disclosure
Service cost	53	48	42
Interest cost	141	140	138
Expected return on plan assets	(182)	(157)	(133)
Amortization of actuarial loss	69	51	54
Other amortization, net	7	6	6
Net periodic cost before deferral	88	88	107
Other pension information
Defined benefit plan assets net periodic pension costs deferred as regulatory asset			34
Recognized for the year
Net actuarial (loss) gain	(20)	(11)	(1)
Regulatory asset adjustment	84	0	0
Reclassification adjustments
Net actuarial loss	10	4	5
Other, net	2	0	0
Recognized for the year
Net actuarial (loss) gain	(307)	(65)	10
Reclassification adjustment	(84)	0	0
Other, net	0	0	(3)
Amortized to income
Net actuarial loss	59	47	49
Other, net	5	6	6
Defined benefit plan, weighted average assumptions used in calculating net periodic benefit cost
Discount rate	5.60%	6.00%	6.30%
Rate of increase in future compensation
Bargaining	4.50%	4.50%	4.25%
Supplementary plans	5.25%	5.25%	5.25%
Expected long-term rate of return on plan assets	8.50%	8.75%	8.75%
Increase/decrease in expected long-term rate of return	0.25
Other Postretirement Benefits
Defined benefit plan disclosure
Service cost	11	16	7
Interest cost	41	45	31
Expected return on plan assets	(2)	(4)	(4)
Amortization of actuarial loss	12	13	1
Other amortization, net	5	5	5
Net periodic cost before deferral	67	75	40
Recognized for the year
Net actuarial (loss) gain	(2)	(10)	4
Regulatory asset adjustment	(4)	0	0
Reclassification adjustments
Net actuarial loss	0	0	1
Other, net	0	0	1
Recognized for the year
Net actuarial (loss) gain	(95)	(164)	64
Reclassification adjustment	4	0	0
Other, net	0	0	0
Amortized to income
Net actuarial loss	12	13	0
Other, net	$ 5	$ 5	$ 4
Defined benefit plan, weighted average assumptions used in calculating net periodic benefit cost
Discount rate	5.70%	6.05%	6.20%
Rate of increase in future compensation
Expected long-term rate of return on plan assets	5.00%	6.60%	6.80%